I think cross-platform Swift is likely to always be a niche like it was with Objective-C. Apple has too much control over the language and too much of a vested interest in retaining that control and modifying the language for its own needs. E.g., the stuff that got shoved in to support SwiftUI with none of the typical review processes until AFTER Apple had already built a shipping framework on it. It was around that time IBM announced it was abandoning server-side Swift as well.