I think it’s important to ask “what guarantees are in place to keep multiple implementations consistent and what is the fallout if they are not” as a lens to determining the degree to which it is or isn’t “world-ending”

The Python ecosystem once fell apart not too long ago because it was supporting two versions where one moved from “print “ to “print(“ and these were incompatible and broke things such as doc tests.

There’s a reason that ppl strongly started advocating to hard pivot to 3: there was a very real chance that Python 2 could fork the ecosystem.

Incompatible concurrency semantics would be a much worse can of worms.