@Edelruth yep, I know. That's two buttons though. My point is, there are probably 10+ keys on a keyboard you never use, as in, literally never. And yet you probably copy/paste dozens of times per day.
One reason I mentioned caps lock is because it's a button most people very rarely use but which has a toggled state - seems like a good fit for press once for copy, press again for paste.

Why don't keyboards have copy and paste buttons?
Seriously, print screen, scroll lock, whatever the context menu button is called? Heck, even caps lock is less useful.