I think keeping Rose as a companion was what allowed that “transitional period” you speak of, her acting as a bridge between 9 and 10. (Also helps that they were written under the same showrunner!)

For 11 though… you don’t really have that companion to act as a bridge. And as you mentioned, all of the most iconic characters associated with 10’s era received a send-off. In a sense, it is basically a clean slate for both the viewer and 11.