Because that's the only scenario where that happens and it's plausible - the exact scenario we were in with IE6 where Microsoft refused to provide a better browser and keep up with then new and much better Mozilla and later Chrome browsers.
Good product doesn't need monopolist lockdown to be used. If Safari as as good as Apple fans say it is, this massive change won't happen. If Safari sucks, then iOS users are better off with Chrome until someone better comes along. A lot of them will probably choose Firefox though, since it suits them better. Or Brave. Or Vivaldi.
It's called competition and drives towards better products for users. It's incredible how anti-free market some of megacorp fans here are. Outright refusing to allow existence of Firefox and other free browsers on iOS is a much worse take than we've ever seen from likes of Microsoft.