As someone who grew up in a medical family, my opinion is that most medical practitioners have a horribly warped view of aging. They spend their working life tending to the worst outcomes of age and end of life care. They think this makes them experts on aging, but really they remain experts on disease.

Plenty of people enjoy life past 80. I wouldn’t draw many of conclusions from your MIL at all.

My grandma is 84 and enjoying life still. Granted she can't walk as far as she used to, and gets tired more easily, but she's perfectly cogent and fun to have a conversation with. She's in a retirement village where she has friends with similar interests, and she even plays ping pong weekly. I won't lie, when I last played her a couple of years ago she could still take points off me. I don't think she regrets living past 80, though of course I would never ask.

My dad is 82. Up to a few weeks ago, he was going on his 3 mile daily walk, etc. He's getting better at playing bridge. He has a medical issue now but treatment may get him back to "full" activity (up to 4 or so years ago that was a 3 mile run, and that's probably off the table forever).

I don't know that he's enjoyed the past year because of COVID not letting him do very much at all, but if not for that, he'd have been golfing, etc, and scowling at the younger people driving carts instead of walking with their clubs :P. I think he has a decent life.