I'd think the motor and rotor itself would be much more likely to be the problem.
Most hobbyist multirotors use smaller controllers (which are essentially a PWM'd set of mosfets) zip-tied or otherwise affixed to the arms of the machine, with short connections to the motors and to the controller/and power-supply distribution board on the body.
Consumer multi-rotor craft will generally have the ESCs integrated on a motherboard or daughterboard in the main body of the craft (cheaper to manufacture), well away from the prop wash and enclosed (not good for cooling).
I'm not sure what commercial (or large-scale) multi-rotor craft use, but it is probably similar to the others.
Again - I'm not saying it hasn't been done, or couldn't be done; in fact, it seems like a good idea. I can even envision how it could be integrated on the motor itself (board on the back with mosfets, with an extended shaft with fan for cooling the mosfets (inrunner motor), or with the fins of the mosfets in the wash of the prop, or something like that for an outrunner motor).
It wouldn't surprise me if such motors don't come on the market (or maybe they already exist - I haven't looked)...
Ok - well, apparently the idea isn't a new one:
https://www.rcgroups.com/forums/showthread.php?1149319-brush...
...and it has been tried and sold to some success. So I imagine such motors are still available somewhere. TIL.
My bet is on the power electronic semiconductors, mostly MOSFETs, in the motor controllers. Life time of these components decreases exponentially with the temperature difference they see in the semiconductor junction, i.e. the difference between hot (current is flowing, heating the junction with a power of I^2 * R_ds_on) and cold.
To make this deltaT_junction small, you need to choose big components, but at the same time you want to save weight, which is fundamentally at odds with overdimensioning everything.