Well, that's true... for almost everyone.
Mixed Connective Tissue Disorder here, pain can be anywhere, but seems to mainly rotate through a few different spots.
One of those spots is my hands, and my thumbs specifically.
I have two e-bikes, I do have a throttle on my hub drive... and I find that it's a little hard on my thumb!
Your general point is really well taken. 64 here, I seem to have given up skiing in 2019, though I still plot a possible return, I'm not sure I'll follow through... I've been on blood thinners since about 2005, and am thinking about quitting while I'm ahead on that one. I can still bodyboard 5 or 10 days a year, but I may have to stop that in a year or two as well. I think the eMTB will be in my future for a long time.
I agree on fit... I do have an old road bike back in NYC, and I do ride it whenever I am back there. But it is SO much harder on my back and shoulders than either my kit-bike hub drive or my eMTB. The eMTB's riding position may be considered 'aggressive,' but compared to the road bike, it's absolutely comfy-- I know that's not the primary purpose of suspension and I appreciate the extra traction from suspension greatly-- I would have dumped any bike without suspension three times on my ride yesterday. But it also reduces wear and tear on my joints.
Riding the kit bike, I find that technique is very important-- it has stem and seat post suspension which helps, but only if I remind myself to hold the bars a bit more loosely while I'm going over cracked pavement or roots.
I ride a very similar distance to you. Eight to 13 miles is my usual ride, three to five times a week when I'm not flaring, with 600 to 1300 feet of vertical. During a flare, I'll only ride three or four miles on the kit bike, avoiding bumps and using as little motor as possible so I get 20 minutes of decent cardio covering a much shorter distance. Flares are weird-- on rare occasions, I find that the conventional wisdom to avoid exercise can make symptoms worse. More commonly, however, it's better for me to play hurt and fight it even if I have to dip down to 90 minutes a week of cardio. And occasionally, I can 'work through' a flare-- if I time it just right, I can shut down the flare faster with moderate exercise. Tricky business, but much better riding vsI