If it were me I'd go on quality of life.
If your horse hates being in and finds it stressful, and you find it helps to manage them when they live out, then that would guide my judgement on whether they would tolerate being in more and reduced grazing / being in a sparse paddock or wouldn't be very happy and it would be better to PTS.
If they seem happy with this kind of routine, can see others next to them when in a stable, are happy watching yard life while they are in and get opportunity for movement when out even if grazing is restricted, then I would try not to project any feeling of being cruel, mean, unfair etc about keeping them in and reducing grazing if that style of management allows them to continue to minimise pain levels and have a reasonable quality of life.
Personally I think horses live 'in the moment' and aren't likely to think about the past when something was different, or be able to compare and have capacity to think a past time was better. If they have enough forage, whether that's hay or grass, as long as they can access that then that should satisfy their need to continually eat.
I have an extremely good doer who lives out. I feel cruel using a grazing mask and restricting her grazing, but I tell myself it's better that than being ill. Where we are is very good grazing, and after a recent injury with recovery that lasted several months to get back to sort of normal, I have found myself thinking more and more that keeping her in a stable and feeding hay rather than grass to have more control over her food would be better for her overall, though she is very happy living out and I know she wouldn't like it as much.
Essentially it comes down to your own judgement and you know your horse best, but I would try not to anthropomorphise or feel sorry for them unnecessarily, as they can adapt well and 15 is not that old (mine is 14). If you are still doing some riding with them then think positively if that's what you want to try, you can always reassess later on if necessary.