Now you jogged my memory...

https://www.wired.com/2009/11/18-button-open-office-mouse-ma...

Also, didn't early mouse experiments involve rodents with many more buttons than the 2-3 that's common today?

Yes, but you only have one thumb covering all those hotkeys. Pressing them in rapid succession will be much easier in parallel on a keyboard, simply because your four fingers aren't occupied on aiming the mouse and clicking the main two buttons.

The one Razer product I own is the Naga Hex, because having the keys 1-6 at thumb reach on the mouse is wildly helpful for RPG hotbars. Not that I think I'm that much faster a player, merely that my WARS movement hand gets a little less to do.

Reminds me that i loved playing the space part of Star Trek Online because i could use the "mouse hand" for hotkeys via the numpad and a couple of modifier keys.