I dealt with the "what to do" question with my kids first mare. After my thinking and planning, she led the process.
When she became too old to keep up with the day to day work, she became the favorite for short rides by us (check water, open or close a gate, get the mail) and took unskilled guests around the pretty pasture in front of the house. When her conformation changed and saddles didn't fit right, but she still felt well, she became the horse that we led little ones on with a bareback pad. When really all she wanted to do was stand a bit, eat a bit, and drink enough, that's what she did. For about 8 months.
Finally, most of her teeth were gone. She was 32. Winter was coming. And there was no way I was going to a) keep her stalled (away from others and risk the health problems associated with that), or b) let a predator take her down and... well, they start eating herbivores like horses, deer and all while still alive!
So I put her to sleep, non-chemically, because of the toxic results on the environment. She was not going to be able to maintain health that winter and condemning her to that would have been unkind.