I think you may be right. As I started riding emtb's, I soon found myself regularly using those mobility scooters at the local Walmart when doing my shopping.
But all joking aside, I don't think there's a one-size-fits-all as it applies to riding an emtb and then experiencing some kind of atrophy in one's legs. Is it "generally" more effort to ride a pedal-only bike over an emtb?...yes...probably...generally.
I say that because there have been studies of top tier athletes and their comparisons to fitness levels. I recall one in particular cited in Outside Magazine that I thought was one of the most comprehensive examples of this issue. It was very extensive in that it measured all kinds of bio/physical categories of fitness ranging from muscle strength, VO2 capability under physical effort, and on, and on. It wasn't just a fluff piece...it was pretty cool.
The somewhat surprising result, even to the study's experts, was that the second place athlete for overall fitness was a world class motocross champion...first being a world class level soccer player. Even in the leg muscle stress tests and accompanying VO2 levels, the MX'er scored extremely high.
But wait...how can that be? He's riding a motorized vehicle where the main use of his legs only "mainly" involves rising up and down on the saddle? Well, those of us who ride dirt motorcycles and even our MTB's/emtb's in an aggressive manner know there is a ton of leg workout involved.
So, am I saying, and does this mean, that riding a pedal-only MTB doesn't provide more leg strength than riding an emtb?...no, not really. However, I doubt the term "waste away" as used in this thread has little application. Obviously how one is riding either type of MTB has more to do with it. If you're just pedaling along in cruise mode on a smooth bike path on either type of bike, then there's probably little difference. It probably has more to do with how aggressively, how frequently, and in what environment you're using either type of MTB/emtb.
Also, I'm thinking the OP is applying a little tongue-in-cheek with this topic, especially since the term "waste away" was used.