While this excuse works today, we should not forget that this policy also meant disinviting Mozilla from the mobile browser party about a decade ago. I'd argue a good chunk of Mozilla's downfall was them chasing the pipe dream of Boot2Gecko, and that was specifically because they couldn't ship Gecko on iOS.

The reason why we have a Chrome/Safari hegemony is because Apple insisted on everything being Safari on their device platforms. This combined with Android shipping WebKit for years meant that the only mobile browser engine that mattered was WebKit. Chrome is a different engine now, but it was forked from WebKit, and it used to have a lot of the same quirks. Hell, Microsoft switched to Blink specifically because Electron - their own web app shell - couldn't run on EdgeHTML.

The fact that this change practically means Chrome displacing Safari is... not really all that meaningful. They're both forks of the same code. The single-engine dystopia you worry about is already here. I daily-drive Firefox, and the amount of shit Google deliberately breaks on Gecko is obvious. Like, YouTube tabs freeze up every few hours because they get stuck in garbage collection, and I have to manually kill whatever processes are running YouTube before I can watch another video. That sort of thing.