If Boris Johnson is the answer, what was the question? To the 92,153 party members who voted for him, the new Conservative leader offers a remedy to two ailments: persistent membership of the EU and shrunken Tory popularity. Applying generous doses of Boris-branded tonic is meant to clear up both problems at once.

About a third of Conservative members were unconvinced by this plan and with good reason. Johnson’s promise to “deliver” Brexit by 31 October cannot be kept. The UK might cancel its European treaty obligations on that date, but there is no closure while the future relationship is unsettled. The less amicable the separation, the harder the negotiations that follow. There are people who believe that Johnson is the best person for that job and there are people who have worked with him and understand what is involved. There is no overlap between the two groups.

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Ministers who witnessed the new Tory leader in action as foreign secretary testify to laziness, inattention to detail, contempt for relationships, congenital unseriousness and dangerous indiscretion. Johnson could not be trusted to stay focused in national security briefings, nor to keep their contents secret. A vote last week to obstruct dissolution of parliament in the run-up to the Brexit deadline revealed the low esteem in which he is held, even on his own side. It was a pre-emptive strike, making him the first prime minister to suffer a defeat in the Commons before taking office. Without a dependable majority it isn’t even certain that he satisfies the entry-level constitutional requirement for the job. A flawed character embarks on a perilous journey from a weak position – it is easy to chart the course to failure.

But Johnson has an irrepressible knack for failing upwards. A shrewd Westminster veteran once warned me against underestimating the phenomenon, observing that “the pattern of Boris’s career has been people predicting that he can’t possibly do something, followed by him doing it.” Johnson’s time in Downing Street might begin in chaos, dysfunction and national embarrassment. That doesn’t mean it will be short. Recent history offers ample proof that incompetent leadership can endure with the help of a ruthless praetorian guard, a regiment of loyal fans and disorganised enemies. Thus blessed, Jeremy Corbyn is still leader of the Labour party. Donald Trump might win a second term as US president.

The charge that Johnson is a flagrant charlatan and has given licence to racists can be true without those traits inhibiting electoral success. His target audience is a coalition of voters who don’t believe it or don’t care. The accusation that he defies diplomatic norms is even less effective. It is taken as a compliment by people who despise protocols and conventions as shackles imposed by a remain-infested establishment. The manual of good governance that Johnson will shred is followed mostly by Westminster’s losers. For that reason it feels prudent to brace for a long stint of bad government.

In historical terms, now might be a uniquely bad moment to experiment with maverick leadership. Britain is adrift between an aggressively protectionist US and a European project destabilised by nationalism. Economic storm clouds are gathering on the horizon. We are only beginning to grasp the extent to which digital revolution is reordering societies and in denial about the consequences for democracy if that transformation is shaped by China. There is a climate emergency. Under the circumstances, it would be handy to have a prime minister with perspective and humility regarding the UK’s capacity for unilateral action.

Instead we have a man whose strategic horizon goes no further than his own ego; who does not distinguish between personal gratification and fulfilment of national destiny. From that perspective, success does not flow from plans, alliances and evidence, but from an optimistic spirit, for which the effervescing source is Boris himself.

Play Video 6:35 'Deliver Brexit and unite the country': Boris Johnson's first speech as Tory leader – video

The complaint that Johnson brings no practical solutions to the challenges he is about to face misses the point. The power of his charisma, on those who are susceptible, is that it dissolves complexity and difficulty. Practical reservations about Brexit can be dismissed as the invention of cowards and naysayers. Any disruption caused by a disorderly rupture from the EU can be embraced as a test of national resolve – re-enactment of the blitz for the benefit of people who missed it the first time and had to endure decades of partnership with a repentant, democratic Germany instead.

Johnsonism is the comfort of parochialism camouflaged in the language of global adventure. It is ignorance of the world as it is, smuggled in bluff and nostalgia for the world as it once was. The psychological appeal of such a creed is obvious. It is anaesthetic against the disturbing reality of the times. The galling element is its claim to patriotism, when a predictable outcome is degrading Britain’s standing in the world and making many British citizens poorer.

But threats to security and prosperity do not have to announce themselves with sirens, jackboots and breaking glass. Decline can be slow. Blame can be deflected. Rogue leaders can win majorities. Westminster has been consumed by Brexit fever for the past three years, but most people are not paralysed by a sense of national crisis. The sight of Johnson on the steps of Downing Street will trigger a range of emotions, including extremes of horror and joy, but in between there is a spectrum of curiosity, indifference, wariness and goodwill. His record is appalling, but he can be unfit for office and effective at staying there. He can be selfish, incompetent and also gifted with the narcissistic gravity that attracts followers. There is no problem facing this country to which Boris Johnson is the solution, but that doesn’t rule him out as the answer people will give when next asked who they choose as prime minister.

• Rafael Behr is a Guardian columnist