I currently don't use a bicycle for practical purposes. I'm "young" and fairly fit and I see no reason to add the cost and complexity of a motor-assist bike to my routine. you can have a lot of fun by going further with less effort with some motor assistance. I won't judge anyone for going that direction, but I would think that leg-powered bicycles would be the default for most recreational riding unless you're riding someplace with extreme terrain or your body limits what you can do on a pedal-powered bike.
however, if/when I have the opportunity to, say, ride a bike to work, I would buy a motorized bicycle of some sort in a heartbeat! if something like that can replace most of the car trips I make, that sounds like a good deal. my current commute (before Covid, working from home now) is about 20 miles each way and takes 30 minutes in the morning and closer to 60 in the evening. I start and end my workday early just to avoid the worst of the traffic. riding a bike, motorized or not, is still a sweaty endeavor in Texas where it's HOT 9 months out of the year, so I'll have to count on using the shower ever day if I try that.
I am seeing ebikes where I live more and more lately and while I think non-motorized pedal bikes more sense, I'll welcome whatever gets people off the couch and onto bikes. when I visited NYC last summer, ebikes were everywhere. getting through a crowded city like that on such a bike made more sense to me and I would definitely have one if I lived there.
mountain biking, on the other hand: I have no need for it. seems like "cheating" to me, for the kind of riding I do, on my local terrain, for me. I know just a few people who ride e-mtbs and it's what keeps them going because they're old as dirt! the idea of riding lots of down-hill terrain in locations where there are actual mountains but no lifts also makes a e-mtb compelling. if you can't shuttle, but a small motor makes the long slogs back up the mountain bearable, that sounds like a good application, albeit a compromise due to the handling effects of the additional weight.