Any fool can knock a coalition. So it proved today. Broken, lamentable, single-bedded, farcical, split, doomed, were among the responses to the "mid-term report" from the David Cameron-Nick Clegg coalition on Monday. What a shower they apparently were.

I suspect history will be kinder. Theirs, after all, was no normal marriage. It was a shotgun wedding, forced on the two parties by the voters 30 months ago, followed by a grisly bout of mutual infatuation in the Downing Street rose garden. Everyone cited Disraeli on British coalitions, and pronounced this one short-lived. I was on the generous side, giving it two years.

We were wrong. Unlike the Callaghan coalition-turned-pact in 1978-9, this coalition has lasted. Two leaders who have serious differences on many topics still stand shoulder to shoulder on essentials and seem likely to do so for a full five years. They have carved out areas of agreement and disagreement and, like political grownups, have seen that they publicly agree on what currently matters. This is mature politics.

Given the state of confidence in the British economy and the state of most European governments, this must have been good for the country. If every measure proposed by the cabinet were subject to a cliff-hanging vote in parliament, like the welfare changes today, it would not make for stability. Government in recession is always ghastly. It is surely worth recognising that Britain has had two and a half years of reasonable consensus in office. Labour's opposition has been left constantly carping, and with little constructive to say.

On any showing, this has been a rare period of political discipline. The cliche that turkeys do not vote for Christmas is easily deployed: an early election would certainly have hurt first the Lib Dems and now the Tories. But this has not stopped coalitions falling apart elsewhere or in the past. Their fate is usually determined by parliamentary morale and the personal antagonism of their members.

There have been minor rebellions aplenty. But there has not been one voluntary departure from the government on a matter of policy principle. Even this week's resignation of the Tory Lords leader, Lord Strathclyde, was said to be over party tactics. We can jeer that matters of principle rarely surface in modern politics. The fact remains that Clegg, in particular, has kept his party in coalition with no serious unrest, despite a horrendous plunge in support at the polls and by-elections.

Credit must go to the personalities of the two leaders. Cameron and Clegg are creatures of the political hothouse. Neither professes any passionate or deep-seated conviction, belonging to rival tribes largely by accident of birth. They are new-style professionals. But they do share one of the style's few virtues, an instinct for survival and a talent for getting on. Imagine how long a "marriage" of Nick Clegg and Gordon Brown would have lasted.

There is no evidence that the fact of coalition has stymied action. Both Cameron and Clegg are reform-minded centrists. The chief curb on the cabinet has been Cameron's lack of intellectual process, seemingly the result of executive inexperience. He acts, then thinks, then forgets, then has to think again. The prime minister has tallied over 30 U-turns. But Clegg has not interfered. He has not stopped Cameron over Afghanistan, over aircraft carriers, over reorganising the NHS and upheaving welfare. He has not stopped George Osborne making the City of London sole arbiter of economic policy.

Clegg is what a psychologist might call a situational appeaser. He has gone along with Cameron's (and his own) U-turn on a European referendum. He has supported Cameron's gratuitous assaults on the English countryside, with planning reform, glamour railways and wind turbines. Clegg must know that each has been a gift by Cameron to Ukip, and since every vote for Ukip weakens the Tories, it marginally strengthens the Lib Dems.

Cameron-with-Clegg has not shirked radicalism. In year one he boldly sought serious review of pensions, health and social benefits, which is more than Labour ever did. The occasional collapse of reform into shambles has been the result of Downing Street taking its eye off the ball, rather than any lack of will. Margaret Thatcher understood that radicalism was futile without total mental application from the top. The coalition's regular banana skins are usually the result of a lack of political direction, command and control, and to No 10's growing obsession with media image, reminiscent of the worst days of Tony Blair.

Just as the coalition may be judged more kindly by history than by the present, so history may see in Clegg's conduct a degree of cunning. Britain's political fault lines are fragmenting. Cameron's critical failure to close a deal with Clegg on 25 extra Tory seats, coupled with his witless aid to Ukip, makes a Labour return to power more likely. But if Ukip votes reduce the loss of Lib Dems seats to the Tories, they must increase the odds on a hung parliament. That, in turn, increases the chance of a Labour-Lib Dem coalition, and thus Clegg's survival in office.

Fragmentation, while it can destabilise coalitions at the extremes, can stabilise them at the centre. Far from being out of power for a generation, Clegg's power-hungry, policy-bereft Lib Dems could have found the path to office for a generation. Germany's centrist Free Democrats have served in government for almost half a century since the second world war.

Ever since he entered coalition, Clegg has been written off as politically crippled, his party as heading for the dustbin. Perhaps we were wrong. In May 2010 we thought Clegg and his colleagues were merely kingmakers for day. Perhaps, after all, they have booked themselves limousines for life.