What comes after ERII will make or break the House of Windsor
What will be history's verdict on the Queen's Diamond Jubilee - another example of the monarchy's uncanny ability to renew its bond with the public, or another example of a class-based elite's uncanny inability to read the writing on the wall?
While the pomp and circumstance were a reminder of the monarchy's pivotal role in British history, the enthusiastic response reflected its successful transition into an institution that sits above the tawdry muddle of politics, impervious to ideology and public opinion.
The Queen is on her throne, and all is right with the world. Yet amid the celebrations, thoughtful monarchists will be privately fretting and republicans will be looking ahead to what they surely see as a window of opportunity.
It's a truism of sport, politics and warfare that your strength is also your weakness.
The monarchist cause's greatest strength is the current monarch and what she brings to the role - 60 years of unstinting service, impeccable conduct, sound judgement and disarming benevolence. But in one respect at least the Queen is no different from her lowliest subject: she won't live forever. And when she goes to her reward it will be, as they say, game on.
Consider the arithmetic. Elizabeth is 86; should she kick on for another 10 years - her mother lived to 101 - Charles will be 73 when he assumes the throne. If Charles were to reign for 15 or 20 years, William would be well into middle age before he gets the gig.
Of course the process could be speeded up. The Queen could step down, although this week royal historian Kate Williams poured icy water on that notion: "The Queen will never abdicate. And if she gets something like Alzheimer's, she will not abdicate then: Charles will be regent until she actually dies."
Or Charles could stand aside for William, as some are already urging him to do. Under a headline identifying Charles as "a real danger to the monarchy", a columnist in the devoutly royalist Daily Telegraph declared this week that "we know far too much about his foibles and past errors to revere him as we revere his mother."
Best for all concerned, therefore, that he bows out: "The coronation of William V and Queen Catherine would be an occasion for considerable hope which would appeal to young people and stabilise the monarchy for another century."
If you look at it from Charles' point of view, such a step would render his life meaningless: he'd be turning down the role for which he's been preparing himself since childhood. It's also quite likely that he doesn't see himself as damaged goods or heir to the tradition of batty aristocrats happiest when messing about at their rural retreats, a real-life version of P.G. Wodehouse's Lord Emsworth, who preferred pigs to people.
The longer William (and Kate) have to wait, the harder it will be to keep the fairytale alive, and the greater the risk of falling victim to the curse of the House of Windsor.
It's easy to forget there was a time when Sarah Ferguson was regarded as an asset to the Firm - earthy, jolly, a welcome counterweight to the neurotic Diana - rather than the weapon of self-destruction and frightful embarrassment that now seems to be her official designation.
Likewise her ex-husband Prince Andrew, once a dashing charmer in welcome contrast to his stitched-up older brother, is now a jowly huckster whose motto seems to be "Have title and passport, will freeload."
The process is already under way with Pippa Middleton. Last year, she was the epitome of Cool Britannia, a style icon with the most divine derriere seen in royal circles since Edward VII (as he became) cavorted with Lillie Langtry. This year she's an upstart on the make who hangs out with the wrong crowd.
People tend to become more conservative as they get older, but I wonder what future generations of New Zealanders and Australians will make of an institution defined by its past and its Englishness and, for the foreseeable future, fronted by people of advanced years and old-fashioned sensibilities. (A question for the organisers: how many old queens does it take to stage a Diamond Jubilee concert?)
I particularly wonder what future generations who don't have family ties to the United Kingdom and who take their cultural lead from elsewhere in the world, will make of the notion which underpins our constitutional arrangements: that a hereditary monarch living on the other side of the world is the ultimate guarantor of our stability and freedom since we can't trust ourselves not to stuff things up.