In her new book, The Palace Papers, longtime royal family expert Tina Brown writes, "In 2011, the question mark over Kate Middleton was whether a girl of less exalted origins could successfully evolve into a future queen. The only question now is how the House of Windsor can survive without her."

Now, eleven years and one day after marrying into the royal family, Brown says that it is Kate who has kept the British royal family from falling apart and that the future of the royal family depends on her "slender shoulders."

"[The royal family] would collapse at the moment without her, I think. Because she is the only modern, beautiful woman who is educated, fulfilled with herself, and actually wants to throw herself into the rigors of this institution," Brown said in Page Six magazine. Not many young women can do that."

Brown says that much of it, promoted by her mother, Carole Middleton, who was "very important" in helping Kate avoid blunders that could have ended her path to a future with Prince William, required "a lot of attention and strategy" to end her marriage to William.

"They had always been madly in love with each other, but ...... from loving girlfriend to future queen, it's an obstacle course," Brown says. 'It's like snakes and ladders. You could step on the wrong square at any moment and get attacked by a snake."

Another longtime royal expert, Robert Jobson, whose book William at 40: The Making of a Modern Monarch (opens in new tab) coincides with the Duke of Cambridge's milestone 40th birthday, Kate a calming force for future kings, and a very positive, down-to-earth influence, she says.

"They have a tight bond and she gives him confidence," Jobson told the Mirror. 'There is no jealousy, no friction, they are happy for each other's success.'