Swift 1 and 2 didn't have to be stable – SwiftUI does. It's a system library, so it can't do things like change method names, at least not without keeping the old symbols around.
Of course, there will be bugs, and behavior will change as the library evolves, but shared library vs language evolution is fundamentally different.
Of course, there will be bugs, and behavior will change as the library evolves, but shared library vs language evolution is fundamentally different.