Not the case for an eMTB. They're physicall they same size as a regular bike.
They're not, though. The bikes are heavier, a lot heavier. And I suspect the riders are heavier on average. Not only that, but they are able to go faster on the trail, and speed + mass is what causes destruction of the trail surface, at the very least widening and increasing ruts. It requires trail builders to build larger radius turns, banks, remove trees where the passage is too narrow for the heavier, faster vehicle to go without hitting the tree, etc.
People get old, get injured, get a disease, and aren't as fit as they were at half their age.
Right, and in ye olden days, they quit riding, and therefore that population of users aged out of the activity, no longer were on the trail. Now the barrier to entry as a mountain biker has been all but removed, or at least replaced with $$ instead of fitness and skill, so more people are using the same piece of fixed real estate. If you simply refuse to understand this, I don't know how I can make it any more clear.
People who are unfit or infirm enough that they can't ride on a traditional MTB would not be on the trail except for access to e-MTBs. So they are on the trail, where they wouldn't have been otherwise, therefore increasing crowding on the trail. And as it happens they are introducing a riding style that creates increased stress on the trail, because even if it is a user who is fit enough to be able to ride a regular bike, they will ride the e-bike faster and it's heavier so it will do more damage to the trail. And if a critical mass of people are this type of rider, then the trail system owners or operators will be pressured to create trails that cater to this crowd and we'll just lose the good old technical singletrack we used to have.
Since you're obviously in the aging-out population, I can see that you see this as a benefit to you. You want to see it as the trail systems are growing old with you. But for the 25 year old version of you, if that person exists today, they will never be able to enjoy the trail the way you did when you were 25. I see this as a significant loss, enough that now I will not be likely to return to mountain biking at all and I never would have even gotten started had it been motorized vehicles on all of the trails back in the 80s.