Martial arts were originally for warriors. ... I have always believed that martial arts were for warriors, protectors, the sheepdogs.
Sure, warriors, soldiers, protectors, law enforcement, etc. But none of those professions necessitate a svelte, trim physique. Of all of them, soldiers are probably the only ones who's profession enforces regular fitness to any great degree.
But all fighting systems are not military in nature. Hapkido was developed as civilian self defense outside of the military. Western fencing (historical, classical, or sport) was developed as civilian self defence for gentlemen and the weapons used had little military applicability. Boxing, kickboxing, wrestling, etc. are also not "warrior" specific.
Then you have sumo, where obesity is kind of a given.
In any case, morbid obesity is certainly problematic for a good number of reasons, but it's problematic mainly for the health of the person themselves and this should not be conflated with McDojo-ism and also is unlikely to cause someone to die of exertion in an encounter that exceeds forty seconds.
One other thought is that while anyone weighing 420 pounds is facing some serious health risks, being exceptionally thin is no guarantee of being in the pink of health. Also, norms of what constitutes being simply overweight shift from time to time.
As far as instructors go, the ability to transmit knowledge and skill is separate from being exceptionally skilled as a technician. A good instructor who has suffered a debilitating injury that prevents him or her from demonstrating certain techniques will find a way to get the job done. Usually via a senior student or assistant instructor. Same goes for someone who's weight, size, or age makes certain techniques difficult for them to demonstrate.