The 'Future of the British Monarchy Belongs with Prince William and Kate Middleton,' Says Historian

"The future of the British monarchy is more about William and Kate—spiritually, not in terms of actual succession," historian Sarah Gristwood tells PEOPLE

Kate Middleton and Prince William. Photo: Chris Jackson/Getty Images

As Queen Elizabeth celebrates her 70th year on the throne, senior members of the royal family are embarking on royal tours across the Commonwealth on behalf of the record-breaking monarch.

While Prince Charles and Camilla, Duchess of Cornwall set off on a three-day visit to Ireland this week, Prince William and Kate Middleton are in the throes of an intense eight-day tour of the Caribbean to three countries that — for now — have the Queen as head of state.

As "the Boss" of the family "Firm," Queen Elizabeth tasked William and Kate with the more high-profile tour for a poignant reason.

Yes, Prince Charles, 73, will reign next alongside his wife, future Queen Consort Camilla, Duchess of Cornwall, 74, but "because of the ages of all the participants, there must be a feeling that longterm, the future of the British monarchy is more about William and Kate—spiritually, not in terms of actual succession," historian Sarah Gristwood, author of Elizabeth: Queen and Crown, says in the latest issue of PEOPLE.

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"The baton of the crown has to pass to Charles and Camilla, but there is a sense of a baton also being passed from the Queen to William and Kate. Charles and Camilla have a lot of life experience between them, and they're not going to change. The future belongs with the Cambridges," she adds.

With that in mind, William and Kate's Caribbean tour involves undertaking the formal work of senior royals, such as meeting the prime minister in each country, as well as achieving what one close source calls "areas of connection" with younger people.

But their visit comes at a contentious time for the monarchy as anti-colonial protests erupted in Belize and Jamaica ahead of William and Kate's arrival, and a movement to remove Queen Elizabeth as head of state of Jamaica is gaining traction.

During a meeting with Jamaica's Prime Minister, Andrew Holness, on Wednesday, Holness directly addressed his country's intention to break away from the British monarchy, telling the couple: "We're moving on and we intend to…fulfill our true ambitions and destiny to become an independent, developed and prosperous country."

For more on William and Kate's Caribbean tour and the surrounding controversies, pick up this week's issue of PEOPLE, on newsstands Friday

Back home, the Cambridges find themselves increasingly center stage as the only representatives of their generation. With the departure of Prince Harry and Meghan Markle from the working family, and the discrediting of Prince Andrew, "senior royals are a bit thin on the ground at the moment," says Gristwood. "Britain is never going to drop all its pomp and ceremony, and it was eventually going to be slimmer and economical. But it's doubtful even Prince Charles wanted it to lose weight so rapidly."

On their overseas tours, "both [William and Kate] display everything that the Queen stands for in terms of duty, of responsibility and dedication and service," says a royal insider.

But as the monarchy faces strong new headwinds, the question persists: Is that enough?