Can endurance be permanently lost by a huge MPC? Shouldn’t it be gained because more effort is put in, so the body will adapt (like lifting weights)?
Oh boy, this is a doozey! My first take on this is to remind you that not every endeavor benefits from 
“muscling up”. I am keenly aware that an athlete comes to understand that it’s critically important to keep in mind what you are training for. No one wants to share the plight of the fanatic, who having lost sight of his goals, redoubles his efforts! As you probably know, there are different types of weight lifting. A body builder uses maximum weight and lifts to fatigue within minimal repetitions. This type of lifting puts mass on the body. But mass is not always desirable. An athlete who needs a sleeker, lighter body type built for speed and endurance (such as martial artists, cyclists, gymnasts and swimmers) will lift lighter weights with a greater amount of repetitions to build in “quick twitch” leaner muscle. Yes, the body will do it’s best to adapt to what it is exposed to. So it’s critical to be wise about what you train it to do.
This brings us back to brass playing. You’ve really hit the nail on the head here, because the body will certainly adapt to the equipment it uses. The question is: will that adaptation prove to be positive or negative?
There are 3 tools that the brass player has available to him to control the flow of air. They are:
1. Opening and closing the aperture
2. Opening and closing the vocal cords
3. Raising and lowering the tongue level.
The ideal playing scenario features a player who can create tremendous velocity of air directed to the front of the mouth while maintaining maximum oral cavity levels. A large oral cavity assures that the sound will be truly resonant in the low and middle registers and allow the player plenty of room to compress the air as he ascends into the upper register. A player who uses a mouthpiece that has an overall internal volume which is too great, will wind up adapting or overcompensating for this disparity by over using these physical tools to create the resistance that the mouthpiece/equipment set up is not providing. So, rather than a player using the above mentioned tools to speed the air, the player will be forced to use these same tools in a way that slows the air, or even shuts it down in an attempt to create resistance (i.e., closing the throat and/or raising the tongue level in the back of the mouth). This is the point of diminishing returns. The player chooses a large mouthpiece believing that this will give him the largest sound possible, but instead, has gone over the edge in that he will no longer be powering this mouthpiece as it was designed to be, but will instead be squeezing and squashing the piece by shutting down all or most of these internal physical tools in an attempt to get the piece to function. Obviously not a positive choice!
In response to your question about endurance being effected over time, it is true that this same player, if he allows this set up to take hold (uses this equipment long term), will have ensured that this physical adaptation will have become his playing technique. He will become a player whose method is to slow and crush the air inside his mouth instead of a player who speeds the air efficiently to his lips…which is the hallmark of solid playing technique.