To avoid a contract with no end date but still have it be a long contract. It seems that the British Royal Family is used because their family tree is public and historically they have lived longer then the average person due do their status.
Disney invokes royal clause in new rules
Under the new agreement, Disney will not need the board to approve new
projects or development rights, an agreement that will be in effect
until “until 21 years after the death of the last survivor of the
descendants of King Charles III, King of England, living as of the
date of this declaration.”
The reasoning is:
The royal clause is used to sidestep rules that prevent contracts with
no end date. The British royal family has traditionally been used as a
reference because their family tree is public and, historically, their
status meant they would live longer than the average person.
By naming not just King Charles, but his living descendants, Disney is
banking on the continued health and long lives of Princes William, 40,
Prince Harry, 38, and their children, Prince George, 9, Princess
Charlotte, 7, Prince Louis, 4, Prince Archie, 3, and Princess Lilibet,
1.
Queen Elizabeth II lived to 96, while her mother made it to 101 and
Prince Philip died just shy of his 100th year. If longevity does run
in the family, Disney’s new rules could be in effect for a long time.
If you put the minimum lifespan of all the living descendants at a minimum of 80 years that means the contract will last for 101 years after signed. If you put the maximum at 101 years it would last for another 121 years. Though it could go higher if the grand children live longer then that.