Have you ever seen a serious "fork war"? Open Source may be possible to fork, but that isn't a guarantee that everything will be hunky dory after a hard fork. The drama and chaos of "we need a trustworthy fork" after a bad actor does something unsociable can be awful (especially if that bad actor remains in play). Security/safety/IP audits of past code pre-fork after a major fork has become necessary isn't free or cheap and takes resources. Drama can draw weird boundaries between project attempts and create a lot of internecine fighting among the "survivors" of the "upstream crash". There's so much sociopolitics that may be involved. Open source projects still involve a lot of people, at the end of the day, not just code. Open source applications have died in a fork war.

The situation is different from IE, but there's still a lot of similarities and open source isn't necessarily the balm it appears to be. They code may still "be there", but code still needs people to believe in it/trust it/work on it.