By saying "a whole new language" I think you mean kotlin since you've mentioned swift as well,

And that's not the topic of this article,

but I'm asking you now why would an android developer learn swift over kotlin?? chances are he'll never work with swift in his entire career because this is not his profession, also speaking of kotlin many people have invested on learning KMM (Kotlin Multi-platform Mobile) recently, So we can hear soon some ios developers start adopting kotlin because it is multi-platform now, but the opposite is not true as I know there is nothing called (swift multi-platform) you can use.

finally, I disagree with you about saying "much career opportunities for Ios"