Earlier quoted context omitted.
Browsers evolve and break older pages. JS that requires requests as you interact with it will need an implementation of the server it uses (the subset and data of it's endpoints) How do you preserve that?
Browsers don't usually break older pages. The only time this happens when you rely on unstandardized features. At least, I have not noticed any page breaking in the past 15 years except for the ones I built using unstandardized API's.
-SameSite:none cookies (with bonus breakage that makes it impossible to use accross older and newer browsers simultaneously without user agent sniffing)
-the planned chrome alert/prompt changes already mentioned below
browsers have been getting a lot more comfortable with the idea of breaking backwards compatibility as of late.