My guess is to 1) stunt the growth potential of their competitors (google, microsoft) before they acquire some sort of generational lead that forces everyone on

My guess is to 1) stunt the growth potential of their competitors (google, microsoft) before they acquire some sort of generational lead that forces everyone onto their platforms and 2) reset a level playing field for a new generation of startups to someday acquire, that are built on THEIR way of doing things.