If more mileage was always better, more milers would be doing 120 mile weeks.
The diminishing returns arent just because of fatigue, they also have to do with how your body responds to a given stimulus. A given stimulus might have more of an adaptation as the length of stimulus increases, but then at a certain point just stop, even if your body can handle more of that stimulus comfortably.
There are a lot of complicates processes involved in distance training, but to very much overly simplify things:
- the more time you spend at a high heart rate, the more your RBCs hit each other and the vessel walls and become damaged and prematurely die. This means you could feel perfectly able to run more mileage, but the increased stimulus that generates more RBC production could actually become neutral (equilibrium) or a net negative (until you reach a lower equilibrium).
- endurance training increases intramuscular stores. It is entirely possible to reach "full" on these stores and not possibly increase this, even with more endurance training.
- all these changes are driven by complex interactions of hormones. Some of these hormones may reach full capacity of the tissues that produce them with a moderate amount of training and nothing non pathological can increase some of these tissues within your body. More training means no additional synthesis, so it becomes the limiting factor in that part of the adaptation.
So even if fatigue wasnt a limit, you might still find a practical limit to mileage for ultras near where most top ultra runners currently train.
Of course, some ultra runners might already be training past their optimum and actually making themselves "slower" by running 200 mpw. However, this might still be their overall best strategy, since part of training is the mental side. Grinding out 200 mpw will seriously mentally burnout some runners, but others come out mentally bullet proof, able to just grind through any brutal race to their limit.