Ever since Britain voted to leave the European Union in June 2016, two things have been clear. No serious attempt to reverse or even moderate the leave decision would ever be uncontroversial. And any such attempt would require a degree of present legitimacy that could be acquired only in the light of new events and with the passing of time. The question today is whether we have now reached such a moment.

Very clearly, there was no such moment in the immediate aftermath of the 2016 referendum. It would have been too soon. It would have been merely a spasm of outrage. The voters had only just expressed their will. No government could defy that decision, even if it had wanted to. Theresa May rightly grasped that she had become prime minister with a mandate to deliver on the vote.

Yet that could never be the end of the matter. Three structural things have ensured that even a stronger and more talented government than this one would struggle to do what May wanted. The first was the difficulty of accommodating any one-off referendum decision within a political system based on parliamentary sovereignty. The second was the general election in 2017, which deprived the government of a majority. And the third was the depth and breadth of the 40-plus years of intertwining of the UK and the EU, which would require years of detailed surgery to separate.

Two and a half years on, things are, unsurprisingly, rather different. The public mood has shifted significantly, though not decisively, against Brexit. Much of that shift is down to the way May has handled the issue – her narrow-minded red lines, her reluctance to reach out – as well as the inevitable wear and tear that begins to affect any prime ministership. A bit of it is also down to extraneous events such as the impact of Donald Trump, the growth of mob politics more generally and the grinding resilience of low economic growth across Europe. Most of it, though, is simply a function of a Brexit timetable that dictates irreversible outcomes after 29 March – and therefore concentrates minds.

Some voters will still be outraged, nearly three years on from the referendum, by the slightest suggestion that any aspect of the government’s approach can be questioned, never mind that the entire Brexit decision might be up for reconsideration in a second referendum. That hostility remains a massive factor that cannot be ignored by those who want a softer Brexit than the government seeks, or who want no Brexit at all. Nor is it ignored. It is the ever-present descant to any and every Brexit conversation or judgment.

‘Theresa May’s admission that the article 50 process might have to be extended is potentially a watershed concession.’ Photograph: Mark Duffy/AP

But the world, and politics, never stand still. It isn’t just treachery, as the cynical French diplomat Talleyrand once put it, it is chiefly a matter of dates. The same thing goes for political imperatives of every kind. There is a time and a place for everything. It was always likely that February and March 2019 would be the time when those who want to mitigate or reverse Brexit would have their best chance of success. It was always probable that these final weeks of the article 50 process were going to provide Brexit’s high-noon moment. And that is precisely where we find ourselves now.

It is natural to be frustrated by the frequent deferrals in this denouement. But you fight a battle only when you have the best chance of winning. That’s true on all sides, whether pro- or anti-Brexit. Sooner or later there must be a battle, or battles. They cannot be infinitely postponed. Brexit must either take place on 29 March or not. If it does take place, Brexit’s terms must be settled in some form or another, or none.

So the fact that this week looked likely at one stage to be a crucial moment but turned out instead to be a bit of an anticlimax should not distract from the main issue. This week, fearing that more Conservatives could join the independents, May managed to buy a bit more time from the Tory party by her important admission that the 29 March deadline may be missed and that the article 50 process may have to be extended if the government loses its plan B vote on 12 March. Buying time is what May does.

Yet the extension of article 50 is potentially a watershed concession. May’s pretence is that this is just a small matter, a little bit of added time because of unexpected holdups. She talked again on Wednesday of such an extension being short and limited. That is simply an unrealistic position. It assumes that the extension would merely be for the purpose of ensuring that MPs finally give their approval to a revision of her original plan and for putting this on the necessary legislative foundation.

Once again, it shows May pretending – as she always does – that nothing would really have changed. Yet in reality everything may have changed. May’s entire Brexit strategy is now being kept alive by two things. The first is her consistent leaning to the right in order to keep Conservative MPs onside behind her Brexit plan. The second is her hope that the attorney general, Geoffrey Cox, can agree some form of legal wording with the EU to enable him to change his advice that the Northern Irish backstop could be a permanent sanction. If he does change his advice, pro-Brexit Tories and the DUP may be persuaded to back the deal.

But both parts of this are smoke and mirrors. More than 100 MPs in May’s party oppose her Brexit strategy. And if Cox gets the form of words that May craves, it will be the only change from the plan A on which May was trounced in January. That doesn’t automatically mean that the Tories who voted against plan A will not vote for plan B either. But it surely means that if May loses on 12 March, and especially if she loses badly, she will have no plan C to put in its place. Extending article 50, in other words, would mean having to embrace a new approach to Brexit.

There are several options here. A second referendum, offering a choice between a deal (possibly May’s) or remaining in the EU might be one, especially now that Labour has shifted its position this week. A more likely approach would involve the European free trade area-style arrangement that cabinet ministers such as Amber Rudd and David Gauke would prefer, though it would involve May trashing her red lines. Or May could try to engineer a general election in the belief that Jeremy Corbyn’s stock is now too low for him to pull off another surprise like 2017’s.

All of these options are possibilities. Most of them would require more than the short extension May mentions – it would be months, not weeks. None of them can be dismissed as complete fantasy. Yet every one is a radical shift from the assumption of 2016 that May’s way was the only way. Partly that’s May’s fault. But the Britain of 2019 is no longer the Britain of 2016. Labour’s shift on a second referendum is part of that. So is the very public revolt by Rudd and her colleagues. The passage of time means the limits of the possible have changed so radically that no Brexit, though not a certainty, is now a real possibility.

• Martin Kettle is a Guardian columnist