I don't really mind the Core team deciding things unilaterally. Language design is not a democracy and shouldn't be. If anything, I think that sometimes too many people try to influence Swift in their own way, leading sometimes to inconsistent language design.
However, in this case, I echo @Max_Desiatov's concerns about the language losing focus. In order to introduce a feature like SwiftUI, it seems that so many low-level design decisions would need to be made first, that the risk of unduly rushing these decisions seems quite high.
To me (admittedly as a non-app developer), it looks like Apple is reprioritising important development resources in order to satisfy some marketing promises, instead of fixing and improving core issues with the language first.
But it's Apple's decision to make, just as well as it's every developer's (or company's) decision whether to want to invest in Swift.