[Chris is me]

Ultimately, all motors contribute is power. To paraphrase the great Dr. Joe, who posted something pretty cool that I couldn't find searching, speed and torque are "accidents of birth" - motors really only contribute power. You can trade speed for torque and vice versa with gear reductions. That's the whole idea behind transmissions and shifting gears. If you want more torque and more speed than you could find at a single balance point, you shift gears.

Shifting actual motors makes a lot less sense. First, when would you ever want less power in your drivetrain? Putting an FP in a CIM's place would just mean you would have to gear it slower to get the same torque. I just can't see a situation you'd ever want less torque.

In addition to that, you're limited to a fixed number of motors, and using more of the "best" motors when you could just shift gears seems a little trivial.