I don't think it would be possible to have one do all. The problem is, for milling purposes, as someone has already stated, the machine needs to be built very ridgid and the gantries usually are heavy because of this, which would effect speed. Because of that, even if you had extra stepper motors to mechanically automatically switch different steppers or servos to operate different modes of the machine for speed/accuracy/ridgidness, those faster servos/steppers would have trouble moving the mass of the gantry that would be needed for the milling purposes.
There's alot of factors to configure. One of which is impossible to get around and that is physics.
You could however built a milling machine that could do routing, laser cutting,plasma cutting, and even 3D printing. One set of steppers or servos could handle that as speed is usually not a HUGE factor, but it won't do P&P because of the speed needed to do that, or accuracy.