It would depend why I couldn't ride. If horse was 'comfortable' and happy being a field ornament I'd probably keep for the rest of their life, if not, PTS.
Difficult one to answer as it depends on the circumstances.
I had one horse who I only had for a couple of months. She was written off LOU, so I sold her as a broodmare. However, my other horse was lame for a year and I wouldn't sell him now that he's sound but not jumpable - although he is now currently out on loan as I'm about to go abroad.
I retired my TB when he got arthritis as he was still quite young and vet said he couldn't be ridden but otherwise in perfect health. I actually found someone to take him on as a companion who I keep in touch with etc but if he had been older or not as healthy then he would've been PTS.
I got a lot of stick for what i did - i was told by several people down my yard that the horse couldn't do his "job" anymore so he should be PTS - as i'm not a serious competer or need my horse to make money from I couldn't see the problem with my decision.
I currently have one who is enjoying retirement. Jesper owes me nothing and whilst he costs a fortune to keep it is my responsibility as his owner to keep him. If he was in pain he'd be pts, however he is currently fit, healthy and enjoying life, it's just a shame he can't be ridden!
Mac's living the happy life retired out in a heard, he's thriving on it and I hope he continues to do so.
At least he's now old enough that I don't have to explain why he's retired anymore.
I did have him insured for LOU and claimed it and used the money to fund his first few years retirement. We called it his pension
We've had two LOU's one we breed from and the other we manage to keep running with regular physio, if that ever stops working though he isn't the type that would enjoy living in the field so he would be PTS. I agree it really depends on what you and the horse is comortable with.