My feelings are you brought the horse home, and it lived its life doing what you wanted, making foals or, or being a companion or whatever it was. I think you are now responsible and owe it a decent life and place to live. Just because she no longer meets your purposes is no reason to just kick her to the curb.
Your first horse you mentioned, was trained you used now that its old and probably not as rideable you just wanna pawn it off on someone to spend money on and care for in it's final years? You may wanna rethink if you should even be owning horses in the first place. Sorry if that sounds gruff but you don't come across as a very nice person that cares about their animals in your post.
You asked what would I do ? I have had old horses, and any horse I bring home I assume he will live out his days forever there or I don't bring him home. I always leave room for the old retired guy, and then my riding horse. Sooner or later my riding horse will become old retired guy, and old retired guy will die. But that's the way it is. I bring them home I am responsible.