I suspect that in the timeline where you’re able to install a browser on iOS with a third-party rendering engine, Apple would make users who choose to install such software endure UX hell such that you should count most iOS users asked to install a non-WebKit browser as iOS users turned away. (That’s to say, I don’t see Apple ever allowing third-party web rendering engines in the App Store, and I don’t propose that they should be forced to do so.)If Chrome was on iOS they would just tell people to download Chrome.
I don’t know many developers, at least as long as they’re in the business of making money, who would willingly sign up for that.