The Crown was evidently a two-year gig for Foy from the very beginning.
Given that Claire Foy (and Matt Smith) apparently already knew that they would be no longer needed after two seasons, their replacement by older actors wouldn’t have come as bad news.
On the one hand, many people might enjoy the job security of signing on for a role that could last upward of seven seasons.
On the other hand, year after year of the same character can evidently be quite hard for an actor on the same show for the majority of a year.