In the event of nuclear armageddon, it is said that the cockroach would rule the Earth. I have a growing suspicion that the hardy insects would find that they shared the post-apocalypse wasteland with Theresa May. She has demonstrated an ability to survive existential threats that has astonished scientific opinion.

This week, she will have endured a year in Number 10 since the general election that was conceived in hubris and ended in disaster. This is not an anniversary the prime minister will want to linger upon. She went into that campaign with a double-digit lead in the polls and Tory dreams of a landslide and came out of it without a parliamentary majority. The consensus was that her credibility had suffered a fatal dose of political irradiation. Conservative MPs counted her longevity as prime minister in days, maybe weeks, months if they were generous. The spectacle of a premiership going to pieces before the nation’s eyes was magnified by a maladroit and frightened response to the terrible Grenfell Tower inferno. In the wake of that, one senior Tory, usually an astute reader of his party’s moods, offered me the prediction: “She is one more disaster away from being ejected.”

He was right to suggest that there would be further calamities, but wrong to conclude that these would be terminal for Mrs May. She had a farcical party conference, followed by a flailing reshuffle that underscored the feebleness of her position. Four of her cabinet have been forced to quit, one of them the effective deputy prime minister. The entire 12 months have been dominated by the joyless grind of managing Tory divisions over Brexit, while her earlier hardline rhetoric has melted on contact with the realities of trying to navigate an orderly withdrawal from the EU.

Yet there she is, despite it all, still in Number 10. In the immediate aftermath of the election, I dubbed her “the zombie prime minister”. Do I now regret that phrase? No. I still think it an apposite description of a premiership that has never recovered its authority. But perhaps I should have added the caveat that zombies can be very resilient. That which is already dead is hard to kill.

From disaster, Mrs May has derived a form of durability. Allies say that nothing else in her political career will match the feeling of abject failure that engulfed her at 10pm on 8 June last year when the exit poll was broadcast. She cried and came close to quitting as prime minister in the subsequent hours. However awful the humiliations that have followed, and whatever further ignominy destiny has in store, nothing will ever be as bad for her as that night.

This is her inner personal fortification. Her outer shield has been the numeracy and the inadequacies of the candidates to replace her. During the year since the election, Tories have found it easier to find ways to describe Mrs May as useless than they have to agree which of them would make things better. One of her more successful calculations was giving a big job to Boris Johnson in the belief that this would expose him as fundamentally unfit for large responsibilities. Amber Rudd was sunk by the Windrush scandal. Jeremy Hunt and Sajid Javid fancy their chances. In common with other potential contenders, they want someone else to arrange the disposal of Mrs May rather than get blood spilt on their own hands. Gavin Williamson’s spider has had better reviews from Tory MPs than its owner. Philip Hammond is said to be planning a speech on capitalism. Philip who? The chancellor has been the invisible man. He has adopted such a low profile that it is not so much a submarine strategy as a deep sea diving bell strategy.

Facebook Twitter Pinterest ‘Philip Hammond is said to be planning a speech on capitalism. Philip who?’ Photograph: Ben Nelms/Reuters

Not so Michael Gove, who is lurking in a rather obvious way and touting himself as half of a complicated double-headed leadership deal with Ruth Davidson. Before she succumbs to that offer, Ms Davidson would be wise to consult Mr Johnson about the perils of coupling her fortunes to the machinations of Mr Gove. Liberal Tories pant for the leader of the Scottish Conservatives to come south to be their standard bearer, but before anything can come of that she first has to acquire a seat in the right parliament. Then a self-described “pregnant lesbian” would have to take her leadership chances with a shrunken and mainly aged Tory membership whose social attitudes lag behind the country as a whole.

These many contenders have something in common: the hunger of their ambition is tempered by caution about timing. There have been 12 months of conversations about replacing Mrs May, but they have all run up against the fear of unleashing a chaotic and vicious struggle for the party that could blow it up. Rivals for the premiership have flinched from that, as they have also shied from taking up the burden of delivering Brexit, an unwillingness to take responsibility for what they willed that is particularly pronounced among the Brexiters. Brexit is the water torture of the May premiership and also its life-preserver. Every Tory who craves the crown has concluded that it would best to let Mrs May perform the contortions necessary to try to hold the party together until it is done.

The hard Brexiters have been exposed as bluffers. Jacob Rees-Mogg is all mouth and trousers, as we say in Yorkshire. If his cabal had the will to do so, they could easily assemble the 48 signatures required to trigger a no-confidence vote in Mrs May’s leadership. The regularity with which the Brextremists have threatened to collapse her premiership and then retreated from the deed has revealed the weakness of their own position.

Labour has played a significant role in the preservation of Mrs May. She has had no better friend than Jeremy Corbyn, an unwitting and unintended saviour of her premiership. In the immediate aftermath of the election, Labour thought she was there for the taking. His admirers indulged themselves with talk of Mr Corbyn being in Downing Street by Christmas. Tories could also visualise a Corbyn premiership – and decided that this was an outcome more to be loathed than soldiering on with a zombified prime minister. Scared that a leadership battle would precipitate another election that would see Labour roll into power, the Tories clung on to Mrs May for fear of something worse.

The Tory party has not revised its bedrock view that Mrs May cannot be trusted with another election.

The Corbyn Terror among Tory MPs has receded since the turn of the year. Labour has become preoccupied with its self-inflicted troubles. The local elections last month did not result in the feared Tory meltdown. Despite everything, the Conservatives have edged into a modest advantage in opinion polls. Mrs May’s personal approval scores have recovered. Her ratings are still deeply negative, but all politics is relative and she bests a Labour leader whose grip on the loyalty of his party’s members makes him irremovable.

None of this means that the Conservatives are going to permit Mrs May to lead them for as long as she likes. Some grudging respect for her dogged capacity to soak up punishment doesn’t translate into a willingness to let her front the next general election. The flaws that were pitilessly exposed by the 2017 campaign haven’t been forgotten. The Tory party has not revised its bedrock view that Mrs May cannot be trusted with another election.

What it does mean is that the Tories are not panicking as much as they were six months ago. They are not sweating as feverishly as they would be were Labour sitting on a substantial poll lead. Tories have decided they can tolerate leaving Mrs May at Number 10 a while longer.

By writing about her survival, I am aware that I tempt fate. Perhaps she will be gone this week. The fundamentals of her position have not changed in the year since she shredded her authority by throwing away her parliamentary majority. She is an enfeebled prime minister in a confined and brittle position. She lives hand to mouth, crisis by crisis. Looming parliamentary votes on Brexit could detonate the Tory party and put her premiership in jeopardy.

She would struggle to survive a rejection by the House of Commons of whatever deal with the EU she eventually manages to come back with. Her premiership will never be better than a contingent one. There is nothing noble or inspiring about this status, but a contingent premiership is still better than the alternative, which is an ex-premiership. Each additional day that she endures in Number 10 is, for Mrs May, a victory of sorts. And an example that it is not only the cockroach that can survive apocalypse.

• Andrew Rawnsley is an Observer columnist