If things continue as they are, and Prince Charles succeeds his mother to reign as king until his death at a ripe old age, then some time around 2040 the young couple getting married in Westminster Abbey on Friday will be King William V and Queen Catherine. By the sheer accident of birth, William will then be head of state of whatever's left of today's United Kingdom. Would that be all right? In theory, no; in practice, probably yes.
If William and Kate behave themselves and contribute to the development of a modernized, slimmed-down constitutional monarchy, this can be better than any likely alternative. I don't think countries such as Sweden, the Netherlands, Denmark and Spain, all of which have monarchs, are worse off than those that have politicians as president. Or would you rather have Buckingham Palace occupied by a President Blair?
With one brief interlude in the 17th century, when English revolutionaries experimented with decapitating one of them, there have been kings and queens in England for more than a thousand years. That's an amazing thing, the stuff of poetry. Imagine Shakespeare purged of all references to kingship. Before you abandon a thousand years of poetry, you should be very certain you'll fare better in prose.
As we see again with the world media invasion of London for the royal wedding, this history, legend and mystique is also a significant contribution to Britain's soft power (the power to attract) and its earnings from tourism. No one goes to Berlin to watch the changing of the guard at Bellevue Palace or to catch a glimpse of President Christian Wulff and the little Wulffs. Most would ask, "President who?" But if you make lots of BMWs and machine tools to export to China, that's okay. Britain doesn't. Instead, it has the Queen, William and Kate.
These arguments from history, poetry and soft power would have to yield if the existence of a constitutional monarchy seriously distorted the democratic process, made impossible an open society with life chances for all, and held the country back in a stuffy past of hierarchy and privilege.
In theory, it does all those things. In practice, however, it does these bad things only marginally, and far less than it did 30 years ago when Charles and Diana celebrated their fairy-tale wedding. There are damagingly undemocratic elements in the British political system - above all, the unelected House of Lords - but the monarchy is not high among them.
According to constitutional expert Vernon Bogdanor, no British monarch has refused his or her assent to legislation since 1707. Some undemocratic obscurantism still results from "Crown prerogative," and the constitutional doctrine that sovereignty resides in "the Crown in Parliament," but lawyer Richard Gordon has shown how Britain could have a thoroughly modern written constitution, firmly based on popular sovereignty, and still keep a hereditary monarch as head of state.
The Queen may have some limited political influence, but there's no evidence she's used it worse than presidents do in other countries. Such presidents can sometimes lift themselves above the political fray, as Richard von Weizsäcker did most impressively in Germany, but they'll always be at least residually associated with a particular party. And somewhere in the past, they'll have had to do what politicians do to get to the top. As a result, some of them will even end up in court on corruption charges. Of course, monarchs and royal consorts can get into trouble, too. But there's less chance of that happening with monarchs, precisely because they don't have to elbow their way up the greasy pole.
Countries plagued by "cohabitation" conflicts between presidents from one party and prime ministers from another must often wish they had a head of state who was genuinely neutral and a personification of national unity. To be sure, this also means you can never have a Nelson Mandela or Vaclav Havel as your head of state. But such figures only emerge in exceptional moments. And there's at least one great example of a European monarch riding to the defence of democracy - King Juan Carlos's decisive role 30 years ago in foiling an attempted coup against Spain's still fragile democracy.
As for the claim that the British monarchy cements the apex of an oppressive pyramid of class and privilege, that seems much less true than it was 30 years ago. In today's Britain, unelected bankers are more powerful than any hereditary aristocrats, and star footballers as famous as any royal.
Kate Middleton's advance to becoming a princess shows that the barriers between the upper middle class - roughly speaking, the 7 per cent of Brits educated at private schools, as she was - and the very top have all crumbled. The worsening problem is in the miserable prospects of social mobility for the majority educated at bad state schools. This is the thing that most painfully sets Britain apart from other modern European monarchies that co-exist with open, egalitarian societies.
If William and Kate are well advised, they'll work toward being the very model of a modern European monarchy. If they don't, or if Charles and Camilla don't let them, then by 2040 the job may no longer be there for them to inherit.
Timothy Garton Ash is a professor of European studies at Oxford University.