Last autumn, Harry was guest of honour at a large private dinner party in London. The evening was intended to boost support for one of his favourite charities but the Prince was apparently in a foul mood. He was annoyed by other guests asking where his wife was and snapped back that she was at home with Archie – which was where he should be. Guests came away thinking that this was a young man who was unhappy and angry and didn’t mind letting everyone know.

Shortly afterwards, the Sussexes decamped to North America to spend the Thanksgiving holiday with Meghan’s mother, Doria Ragland. They stayed there until last week when they returned to drop their bombshell announcement.

For those who were at that dinner, it was startlingly obvious that something serious was up. Either Prince Harry was going to have to find a way to come to terms with his life, whatever that meant, or he was going to have to make a dramatic change.

Guests came away thinking that this was a young man who was unhappy and angry.

There were rumblings that the relationship with Meghan wasn’t going well, but then there often are when you get a group of old pals together discussing the addition of a new bride to the group. The more charitable guests put his dour behaviour down to parental sleep deprivation.

Well now we know. Or more accurately we know something. Because the truth is that what we don’t know far outweighs what we do about why the Royal couple have made a decision that on the surface is fraught with so many seemingly incompatible and frankly naive expectations.

There is no doubt that the main force behind the change will have been Meghan. She is a driven American woman who comes from a can-do, kick-ass culture. Harry was brought up in a world where there is always a Tommy Lascelles-like figure (for those not watching The Crown, he is the slightly sinister courtier wheeled in to tell everyone what they aren’t allowed to do) hovering in the wings.

That things must remain the same will have been the default position throughout Harry’s life. Meghan will have found that kind of reverence for the status quo (a status quo that has generally served the Royal Family well) deeply frustrating.

The result is that the couple now wish to exchange their life as ‘senior Royals’ with all the privilege, financial security, respect and, in the main, adoration that brings for the less certain limbo of a halfway house – with one foot in Windsor and the other in the smoke and mirrors world of international celebrity.

There is no doubt that the main force behind the change will have been Meghan.

What is baffling to me is why they are choosing to swap the unique and deeply rooted status they currently enjoy for the capricious and transient position of celebrities on the open market. The greatest aspiration of many of the world’s best-known and wealthiest stars is precisely for the life Harry and Meghan already have – the palaces, the jewels and gowns, the holidays, the staff, the deference and, crucially, the unassailability of their position in the pecking order of fame. What wouldn’t David and Victoria Beckham give to be a royal duke and duchess?

What, me worry? Prince Harry in 1992, when financial independence was just a gleam in his eye.

The Queen has ordered the many different Royal households to get this chaos sorted – pronto. To find a way that her grandson and his bride can assume some of the independence they crave while minimising any reputational damage it might cause The Firm.

Private secretaries will be working overtime to come up with what will amount to a contract that will tell Meghan and Harry what they can and can’t do in their new world, and what they will receive in exchange. Details such as whether they keep their HRH status, when and how they can use it and how much they will receive from Prince Charles.

That contract is going to have as rocky a ride as the Brexit withdrawal treaty. The Sussexes aren’t going to want to be ‘told’ anything, and probably won’t like what they are told. We saw, in the brusque way they ignored both Prince Charles’s and the Queen’s requests to delay the announcement until more details had been smoothed out, that they can be stubborn as mules. But negotiate they will have to because royalty isn’t – and absolutely can never be – for sale.

If their intention was simply to disappear for half the year and live in a Vancouver wood, showing Archie the native chipmunks on pre-breakfast hikes, that would perhaps be disappointing for their family, but not any kind of serious problem. However, disappearing is not what they are intending. Not at all.

That contract is going to have as rocky a ride as the Brexit withdrawal treaty.

They, or more particularly Meghan, will want to be out there. One of the aspects of Royal life she will have found most difficult is not being allowed to voice an opinion. She has forged an identity as somebody who wants to make a difference in the areas that she chooses – female empowerment, under-privileged communities, racial diversity – and she wants her voice to be heard.

She is sincere. She is very good at it. Everyone involved with her initiative at Smart Works, a small charity that helps disadvantaged women get into the workplace, was immensely impressed by her input. Her contribution went well beyond making a token appearance and showing an interest. She came up with the idea of designing and selling product to raise funds and profile and gave an inspiring speech without notes at the launch.

This is a woman who knows what she can achieve and what she is worth. As Meghan Markle, B-list actress in a successful TV series, that calculation was relatively straightforward.

For Harry, as Duke of Sussex and sixth in line to the British throne, there was similar clarity. But as second-tier Royals roaming the world on the speaker circuit, making red-carpet appearances, endorsing products and occasionally appearing on the balcony at Buckingham Palace, the rules of engagement become very murky. Right now the Sussexes are hot.

Meghan is beautiful and impassioned. Harry still dashing and a frontline Royal. Things change. In the world of celebrity, you are always having to watch out for the new kid on the block. Fame is a greedy beast that has to be continually fed.

Until now, their working life of Royal tours, walkabouts, visiting disaster zones and being guests of honour at charity events has automatically fuelled and burnished their profile. But without the very special imprimatur of first-tier Royalty, the kind of power and influence they currently enjoy may not survive in the long term – especially if they are cashing them in to gain financial independence.

The celebrity circuit is complex and quite brutal. If you become known as an easy gun for hire to the highest bidder, in a short time your currency drops. You can spend a few years giving hugely lucrative speeches for international hedge-fund dinners, and being on the advisory board of a biodiversity start-up in Qatar, but to give this existence longevity you have to keep replenishing your worth – whether that is in the degree of your fame or in exceptional talent or knowledge.

Unanchored celebrity has a built-in obsolescence. It’s hard to understand why they are swapping a profile that confers automatic and eternal worldwide fame for one where your worth will be in part measured by your number of Instagram followers and dependent on the skill of your marketing team.

The kind of power and influence they currently enjoy may not survive in the long term – especially if they are cashing them in to gain financial independence.

This kind of celebrity is also reliant on exposure, and not simply the exposure that you selectively chose to put out on your own social media sites. The relentless commentary of media of every kind is part of the deal and, if anything, the Sussexes may well be less protected in this new world than they felt in their old. Every move they make will be analysed and scrutinised and if they are being paid handsomely for it, they will no doubt be regarded as fair game.

The couple are no doubt exhilarated by having made the decision to cut loose and get their plans out in the open. Almost certainly, they will be looking at the future and see it glittering with opportunity. Which it could be. If they don’t take their position for granted.

But if they are hoping to build a new life that trades on royalty without fulfilling the more difficult obligations that come with it, which is reliant on the toxic mirage of fame without worth, not that many years down the line they could be looking at a very different picture.