Except for the very rare horse who is miserable living outside, I don't see any problem (other than the modest financial cost) with keeping a pensioner at pasture. That's what my horses are -- well one of them is only 10 but he is de facto a pasture pet since I got him before I realized that with two young kids and a whole lot of house repairs and so forth I would never really have time to do anything with him -- the other two are 23 this year and live out 24/7/365 (except for the occasional really nasty night when I'll bring them in for a while) and seem quite happy. (One of them cannot be stalled due to mild heaves and severe arthritis). And they will stay here with me, at pasture, for the rest of their lives, however long that may be. I owe them, and they have no apparent complaints about the situation, they are enjoying themselves (and have been for years now).
OTOH if finances are an issue and it is simply not *possible* to keep the horse as a pensioner, or if despite a good long transition period he remains really miserable being kept that way (a companion often helps), then I can certainly see considering euthanasia. I have often thought about what would happen if I had to leave my husband and obviously would not be able to take the horses with me, and unless I could find them a good solid *dependable* living-outdoors home before they became literally homeless, I do think it would be a lot kinder to have them euthanised (at least the older two) rather than have them go somewhere Bad or run thru a sale barn. Not necessarily true for all horses, but for these two it would be, IMO.
So yes, difficult decision.
Best of luck,
Pat