Is going to exhaust your thumb and cramp the hell out of your hand in record time.

And on top of that, occupy some 'third' control scheme somewhere between "kb/m" and "gamepad" that developers would have to explicitly target, at a time when so few of them even put the time into adding proper kb/m support for their console-first-PC-port-maybe-if-its-cheap-to-farm-out titles.