I would say there’s a massive difference between maintaining a mule corps so that in the rare event you want/need to transport heavy supplies through terrain where vehicles can’t go, you have the option, versus using mules as an alternative to vehicles.

The former gives you an element of surprise and/or the possibility to resupply an otherwise cut off unit, much like the capabilities granted by special forces. The latter is a sign that you don’t have enough vehicles, and that your industrial base isn’t able to keep up with losses.