It's the slow march to unification across macOS/iOS/tvOS. Last major update they made "zoom" take the window full-screen in its own space by default. This major, they move towards apps grouping documents under the same window. Next major they improve spaces support (potentially making an application space run in a window), then they've migrated the window-based-paradigm into an app-based-paradigm (like iOS and tvOS). iOS apps already support flexible sizing since iOS 9 to enable this. Once applications can (optionally) support multiple input methods (touch / mouse / keyboard), all new apps are already using the same underlying tech (CloudKit, UIKit, etc.) across all apple OSes, and iOS apps bytecode format already runs on macOS. Respect to them for exercising restraint toward unification to let their users catch up, as opposed to going too far too fast (windows 8) and alienating everyone, but yes some people will still be alienated. No one can argue "system integrity" AKA Rootless doesn't improve security, but it can't be argued it doesn't disable some use cases.