I hear this a lot. I don't really see Google getting fed up with flutter, due to what they have invested in it, but for the sake of argument let's say that did happen. Google abandons flutter. What happens next? Will it fizzle out?

To take your point about the domains, flutter's, and by association dart's, domain is cross platform development from one codebase. As far as I can tell nothing else comes close just now, that domain makes flutter relevant no matter what. Surely as opensources projects, both survive?