As I've mentioned here previously, I'm childless but maintain a slew of bikes for my extended family. We have build two major trails locally and we stated a bike club and have run through well over 100k into our local partks/trails. But anyway, given I'm 68 I tend to need a lot of downtime/recovery but also very much need vigorous effort exercise. I find without a doubt that riding my hardtails (we have Epic and Chisel versions) for a 45-minute to one hour jaunt, with lots of standing pedaling - well, this is incredible exercise for an older person. Zero injury, zero next-day pain or soreness, and lots of abdomen, thigh, torso exercise that keeps you able to to all the other things in life.
Sitting on the SJ? Yea I sit down and pedal those bikes too especially on longer rides with others where if I took the hardtails I'd be worn slap out. But on the day-to-day thing I'm always going for the hardtails which to me are the most simple and well-designed exercise machine available.
It isn't always about avoiding this or that, sometimes it is about "I want the most lively thing I can get." And that my friends is riding a 29er hardtail like the Chisel. Super bike bar none, and not expensive.