Eric.
Your question as I am sure you know has little to do with backpacking per se. I am not privy to any land management agency’s discussion of this issue today, But I imagine it is akin to the discussion 14 or 20 years ago about allowing MTBs on trails. Here is the San Francisco Bay Area including Marin County which claims to be the home of MTB the land management issues in my recollection revolved around impact and access. To take the first issue, impact. One person on a mountain bike can travel further in less time than a hiker or even a equestrian. In winter, when the trails are muddy the mountain bike traveler (and these days they are often in groups) can have a greater adverse impact on the trail than a pedestrian. But many of the land management agencies also have a mission to promote access. So the outcome was to allow MTB access to certain trails, some on a seasonal basis.
Now if we apply the same criterion to electric motor assist bikes, the impact by any one user will be substantial because the user can travel further with less effort not to mention that the bike is heavier. The allowance of the use of the electric assist motor MTB could in in theory allow greater access by the senior cyclists. But on the contrary side, you could argue that the senior should just be fit enough to ride a real MTB.
I would expect the greatest opposition to electric assist MTB on MTB trails to come from the MTB community itself.
Having rented electric-assist motor MTBs in Europe, my concern would be the skill of the riders vs the safety of other trail users when the electric assist rider is coming down hill because the electric bike can travel at a greater speed with less work by the ride and is twice or three times the weight.
To conclude, in my self admittedly non expert opinion, the electric assist bike on MTB trails takes more skill to ride safely downhill on single track. But the very fact of the electric assist enables less skilled riders to venture out on the trail.