“The state of our union is strong.” Those are the words uttered with ritual regularity by a US president delivering his annual address to the nation. What, though, is the state of our union, nearly a century older than the Americans’ and questioned this week as never before? You could say it has emerged from yesterday’s vote in Scotland with its strength renewed, reaffirmed not by the whisker foreseen by the closing opinion polls but by an unarguable 10-point margin. What’s more, after weeks of speculation over the future of David Cameron, it was the advocate of independence, Alex Salmond, who resigned today. Earlier, one of his lieutenants had admitted that independence would now be shelved for a generation.

But the relief of unionists, like the heartbreak of yes campaigners, should be tempered. For there is another way of looking at the verdict that came as Thursday night turned into Friday morning. Independence used to command the support of a stubborn third of Scots, and no more. Yet yesterday 45% voted to repudiate British sovereignty, to end this arrangement once and for all. When close to half the population of a nation inside a union wants to break away, the state of that union is not strong. It is fragile.

A wafer–thin victory would have been described as a constitutional crisis, a crisis of legitimacy. But when 45% want out of a state, that is perilously close to the same thing. Even those unionists celebrating that a majority chose to stay with the United Kingdom have to concede that this result will bring as much turbulence as resolution. For the union’s victory was secured at a steep price, in the form of the last-minute, cross-party “vow” by the three Westminster leaders to cede more powers to Scotland. There were plenty of Tories who looked at the 10-point winning margin and concluded that, in his panic, Cameron had overpaid: he could have won by promising much less. But the prime minister is stuck with it now, honour-bound to implement a series of pledges scripted for him by his predecessor and one-time nemesis, Gordon Brown. Cameron’s 7am address in Downing Street today reaffirmed the pledge, leaving himself little or no wriggle room.

If he tries to escape his commitment – as Salmond, in his resignation press conference, suggested he might – what little faith remains in him and the entire Westminster system will be shot. He will have confirmed the nationalists’ most damning verdict on the UK parties: that they are liars and cheats who will never deliver for Scotland. Break this vow and neither Labour nor Lib Dems nor Tories will be believed again. Salmond is right that after such a monumental betrayal, Scots would be “incandescent”. They certainly wouldn’t wait a generation to break away.

But if Cameron makes good on the “devo more” promise, he will stir a new pot of resentments. Note the speed with which Conservative MPs rushed to insist that any new powers to Scotland had to be matched by equal autonomy south of the border. It was an almost comic exercise in solipsism: these Tories heard the Scots’ cry for a new beginning and decided it could mean only one thing – more power for England. In other words, whether Cameron keeps his word or breaks it, upheaval is coming. In that sense, the constitution is in turmoil even if it is not in crisis.

Radicals tend not to be scared of that latter word. On the contrary, they welcome it. They remember the maxim that one should never let a crisis go to waste, for they are opportunities to make great change – in this case the chance to reshape a state designed for an imperial age long gone. But neat though that maxim is, it is hardly reliable. The dispiriting truth is that crises often do go to waste. Things fall apart but somehow the centre holds, against all expectation.

The exemplar is the global financial crash of 2008. It seemed impossible to imagine that having pushed the world economy to the brink of collapse, the banks might simply be allowed to revert to type. But revert they did. Despite the fevered anticipation of sweeping reform, if not the remaking of capitalism itself, Wall Street and the City soon returned to big business as usual. It seemed similarly obvious that the MPs’ expenses scandal of 2009 would turn Westminster upside down. It’s true that established politics remains distrusted and, as both the yes campaign and the rise of Ukip testify, despised partly as a result of those revelations – but those who braced themselves for dramatic change are still waiting.

Even moments that are not crises, but that trigger great moments of public engagement – moments that seem destined to leave a lasting mark – can instead evaporate. The intense, febrile week that followed the death of Princess Diana in 1997 was surely going to force a profound change in the nation’s relationship with the monarchy. But the heat faded and the moment passed.

Right now, even the most cool-headed Scottish historians and analysts are insisting that, thanks to the yes movement and the phenomenal civic energy it unleashed – confirmed by a turnout of 85% – Scotland will never be the same again. “The genie cannot be put back in the bottle,” says one. Trouble is, our system is very good at doing just that.

The current Tory reframing of what happened in Scotland as being, in fact, all about England is a choice example. What better way to choke off a popular Scottish awakening than by giving it the face of Bernard Jenkin and Nigel Farage. The process now under way is similarly deadening, making it a partisan battle between the Westminster parties, the Tories hoping to score a point over Miliband by daring him not to join them in insisting on English votes for English laws. Less cynically, the risk to this moment is that it descends into the arid abstractions of “constitutional issues”, a movement of the streets turned into an anoraks’ debate over regional assemblies, voting systems and block grants. Those Scots who had never engaged with politics before now would rapidly turn away, their fellow Britons with them.

If what started in Scotland this late summer is not to disappear by midwinter, it is its spirit that has to be nurtured and replenished. Simply defined, that spirit was the determination to ask people a profound, even existential question: not to trap them in the weeds of this tax cut or that benefit change, but to ask them what kind of society they want to be. Such questions need not be only about identity. Sometimes, as perhaps in Scotland, they can be about both identity and the desire to live under a different, more just economic system. If this extraordinary moment is to last, that’s what we have to remember: that if you want people to come up with the biggest answers, you have to trust them with the biggest questions.