"Forced" is a bit strong. They did the math and realized that being competitive in the browser market was not worth the $$$$ they would have to spend to get there. Given that web browsers aren't really going to make MS money, unless they manage to get full dominance (which isnt really their strategy anymore and would be difficult in modern context), it was probably a smart business decision. If the business calculus was different i am sure they could.
Google did actively slow and cripple its flagship sites on competing browsers without suffering any consequences. No amount of money will help against that kind of advantage.
I can confirm. We have a normal Google doc (nothing super fancy) that spans about 10 pages at most, and when I scroll past about 8th page in Safari, scrolling becomes almost unusually sluggish and everything starts lagging.
Don't know, maybe not. It works flawless in mobile Safari, for example. Even though it's not intentional, I'm pretty much sure Google would be more inclined to say "hey use Chrome" instead of fixing it for Safari.