Loan, I guess. Or keep them on the very cheap (grass livery, no facilities, sell tack etc). I would have my old lad put to sleep before I sold him or loaned him out for anything other than hacking though.
If my mum and / or sister couldn't take him then I think I would have him PTS. he doesnt mind things changing as long as I am there with him (as Ive had him so long) although he is happy with mum and sister looking after him a short time I am his mum, he wouldnt cope with someone looking after him full time as he is used to my ways and is a bit nerdy He has also been at his current home (mums yard) since 2001 hes moved away for a couple of months with me and loved it but long term he wouldnt be happy 
I have one sharer (who I'm about to lose as she's moving to Wiltshire) and one previous sharer (who also moved to Wiltshire, I don't like that county!) who both wanted to take my boy with them when they moved (although they knew I'd never agree!) so I'd turn to them first. I'm sure one in particular would have him (they're not close enough to share him) and I'd help out financially as much as I could (I'd like to think I'm secure enough for finances not to be a reason for struggling to look after them) so I've not thought about it past them. It's in my will that he goes to one of them with £5k to look after him for the rest of his days. She couldn't sell him, but she could rehome him with the agreement of my best friend (and remainder of money goes to charity) - not that I don't trust sharer to make that decision, more because a collective decision about a new home will take pressure off both of them. They can be happy that someone else has vetted a new home as well.