And while it hurts now, it's REALLY going to hurt when large swaths of useful answers that don't exist anywhere else are gone and there's nothing replacing them.

Noone writes hundreds of pages of documentation for their stuff anymore. Without the collected knowledge learned from experience there, what do we have?

Unless we have source code to read, very little.

I'm still feeling the pain of google search results sucking combined with most of the large coding forums being gone and reddit slowly going to garbage. Stack Overflow was the last bastion of collected knowledge of it's type... and it's not like it was 25 years ago where we still had phonebook-sized manuals for almost all major software because agile has killed the concept of exhaustive definitive documentation for a given version of something.

I used to sorta roll my eyes at people shouting about federating everything, but at this point I'm scared and agreeing with them.

Vandalism is always reverted on SO, even if done by the original author. No knowledge is lost. Suing OA for violating the CC-BY license might be possible, but I'd wager SO is not interested in suing them, and since they hold the rights, not much can be done by others.