The problem I see with this spec (and the current implementation in browsers) is that you can't trigger a copy without having the user manually press ⌘C or do t

The problem I see with this spec (and the current implementation in browsers) is that you can't trigger a copy without having the user manually press ⌘C or do the platform equivalent command for that. That way you can't implement custom UI (like toolbars) or even have a link automatically copied to clipboard (by a URL shortener, or by a file upload website). This is specially problematic on iOS and Android, where copy and paste depend on contextual popups/bars that only show up when you have text selection. This makes it much harder or impossible to implement advanced editors like say, a 3d modelling application, or image editor with copy and paste functionality on these platforms.