MTB and the 'energy' involved is much different than what is encountered in a long, fast decent on a road bicycle. PLUS enclosed reservoir, PLUS smaller caliper, smaller rotor, less fluid.
NOT 'assumptions' but what the shimano and sram testers have found. When shimano test guys 'test' road hydros, they wear full body armor like downhill MTB guys, because more than once, all the brake go away and they have to crash to stop.
shimano has had Di2(lots of room in the lever) and wet discs for how long? Why do ya suppose wet road brakes haven't been introduced, if it's all so easy?
Besides, disc brakes on a normal enthusiast's bicycle is just plain dumb anyway. More expensive, More complicated, heavier, requiring major modifications to the frame and fork. An answer to a not asked question and completely ignores the 'whys' disc brakes are on MTBs. Ya know, wet, sloppy, chance to wack a wheel, type stuff.