The threat isn't
just that Google does something intentionally evil.
Imagine if there's a WebKit 0-Day that Blink inherited discovered tomorrow that breaks ~96% of today's browsers as soon as that bad actor discoverer weaponizes it. (Or worse uses it to send malware or as an RCE vector.)
That's not something Google did intentionally. That's not something that forks can "immediately" fix. That's something that possibly forks make worse because everyone is going to be auditing different but similar codebases for the same bug and there may be some political infighting and finger pointing and accusations in a mexican standoff between the forks all accusing each other.
Most importantly, that is a lot of potential devices at major security risk to a single ecology "family of forks" of a code base.
What other piece of software does something like 96% of internet connected devices possibly have in common? We've got a variety of operating systems (Windows, Linux, macOS, iOS, Android, etc). We've got a variety of hardware stacks. We've got a variety almost everywhere but this one massive bottleneck growing worse. There's still some hope in the case of the hypothetical WebKit 0-Day that that diversity alone is enough to keep things from getting truly bad, but if you believe in software security you've probably had "defense in depth" drilled into your head and does "everyone uses the same browser codebase" sound like defense in depth?
That's the biggest nightmare scenario. There's lots of little iterations of that, including all the various little ways that standards might slowly get ignored or broken until some day the future web is wondering where the standards specs even are and can't find its way out of the box it was trapped in. If it happens little-by-little enough ("boiling the frog" as the aphorism goes) no one sees it as a clear threat at the time and no one thinks to "immediately fork it" until it is already far too late. To repeat something that needs repeating a lot in these conversations: the problem with IE6 wasn't that it was terrible and "behind", the problem with IE6 was that it was amazing and forward thinking and had boiled some frogs building specifications too quickly ahead of the standards bodies. People didn't notice the problems they had from that until after they'd built websites and apps with IE6 in mind because it was great at the time. People didn't notice the problems until after Microsoft felt they had done enough and took their ball and stopped playing because they had "innovated enough". People mostly didn't notice the problems until it was too late to get out of the box they'd been trapped in without realizing it. IE6 today has a reputation as a terrible browser that was stuck "behind" standards, but that's not where it did the most damage. It did the most damage when it was "the best browser to use", "the most innovative and powerful browser" and "everyone uses IE6 and is happy with it".