What market dominance? Afaik, Android has for more users in most parts of the world - Apple just dominates iOS ;-)
But here's why I would not keep SwiftUI an exclusive technology:
Cross platform is definitely desirable for many people - we all may agree that it's crap, but managers love the concept of doing work once and selling it twice ;-)
So if SwiftUI would be a serious alternative for platforms other than iOS (and Mac... although most people seem to forget that most of the time), there would be some pressure to use it - not because it's the best way to build apps for Android, but because it is good enough to do so.
For Apple, I think this would be beneficial, because it would be their platform that has the best integration, whereas the other platforms would have all the apps as well - but the overall experience wouldn't be as good.
On the other hand, what if some alien alternative turns out to be "good enough" to build apps for iOS? I think this would have bad consequences for all of us (Apple as well as Swift developers).
Of course, all this doesn't matter at all, and probably nobody with the power to decide such things will ever read this threat ;-)