I have found more of my issues with the wiring and terminals and messed up clip cords than I have with the motors. I also have machine designs that put minimal strain on the motor. I am sure that plays a big part...and to add to that, my motor problems have been with the larger ones, not the smaller ones. -go figure :-/
I have had minimal issues with actual motors in the past year..so I'm not too concerned.
also, I have a hard time believing that a Maxon motor is going to outlive a Mabuchi motor long enough to warrant paying 10x more for it.