I have 3 horses, only 1 is ridden. If he couldn't be ridden anymore for any reason I would miss it but it wouldn't change how I feel about the horses and what I get from just being around them
Yep as long as they get enough stimulation I think it's nice
One of our old boys did get bored in retirement so I would take him for walks (felt more like water ski-ing to be honest) and make little obstacles for him to loose jump. The other was in a much bigger field (like 25 acres) with hills, some forest and a stream and he was much more content.
Both of mine are pets - one is elderly and retired, and the other is slightly less elderly but broken, and most likely will remain that way. They're happy as Larry and so am I. Even before my small boy got his injury, I found myself riding less and less. At the age I am now (not ancient, but sometimes feeling it), pootling around with them and my dog is just fine.
Both of mine are currently pets as I'm injured and won't ride for another 6 months - they are turned out 24/7, are right in front of the house so see and are involved a lot in what goes on, get lots of attention and will happily lunge if I ask them too (which is entirely dependent on whether my leg is sore). They both seem happy enough and are very relaxed. It's me that can't wait to get back riding!