Sharing middle click between paste functionality and it's other popular functions makes it error prone and impractical at least for me. Copy paste is faster in a vacuum but the overall experience is worse. I think using thumb mouse buttons is more practical, and auto copy is better just avoided altogether.
Maybe that's why the "Un*x dogmatists" avoid adding functionality to mouse buttons. Middle click already has a great X11/Wayland feature and should stay that way.
I may have something on my clipboard already. I "compulsively" highlight things as a reading aid and it's frustrating when I lose trust in my clipboard because it always contains fragments of text I don't care about.
I'm somewhat sympathetic, but I'm not sure that "I want to be able to give the machine input without it reacting" is a great objection to doing something with that input.
Autoscroll is great by the way.