This is interesting to me! I have a couple of disjointed thoughts about it so I'll do my best to organize them!
I think that this is a genuinely valuable topic of conversation!
I wax back and forth between agreeing and disagreeing with you on the sentiment of just letting "disabled" horses go. This is something I've really waffled over since coming more to terms with how truly disabled my gelding is.
On the pro side, I learned mid-summer 2016 that my gelding actually came out of the kill pen in September of 2012. Someone stuck him in there [illness forced the sale] and despite the horse numbers in that sale reaching an all-time low [90% of the no-sale horses went straight to kill], my gelding got a second chance in the pen. Someone pulled him out ON his ship date [mysteriously, his ship date was actually my 21st birthday], he was resold a month later with a false story, and ended up coming to me about a year later. I only found out about his kill pen adventure when I randomly mashed together the right search terms in Google and suddenly a picture of MY horse popped up in an archived auction listing! Talk about a surprise.
Anyway, I still feel like, with his issues, maybe being sent to slaughter would have been a quicker, and therefore nicer, end than his current life of variable, never-ending pain. If he had gone then, I don't feel like it would have been wrong. I don't love slaughter, but I think that that perhaps is a good reason for it - an out for horses that need an out.
NOW though, now that my gelding has made it this far, I can't imagine putting him down for his issues. He has already escaped death numerous times and he has so much try and hope, I couldn't justify letting him go right now. I have a pact with all my animals - when they are uncomfortable more than 50% of the time, when life is more of a challenge than it is enjoyable, I let them go. My job is to steward them, not to hold them here, and I take that job seriously.
Additionally -the thing that really gets me- what if my guy is the one that has the genetic key to his disease, or what if, at some point in his life, we figure out a treatment that works and ends up helping tons of other horses?? He's one of the younger ones, that I know of, with his particular symptoms - he could give science all kinds of information about longterm care, what works/doesn't work, lifespan, progressive-ness of the disease, and so forth.
I seem destined to get "broken" horses, I might as well stick with one I really like!
There's also the factor that the research group we're working with has, based on statistical analysis of test results and associated bloodlines, estimated that 20%+ of AQHA horses are likely to be affected by one or more PSSM myopathy [P1, P2, P3, Px, P4, and/or P5]. The statistics are similar with Arabs. Positive horses have been found in numerous breeds, many continents, all riding disciplines. Those statistics could change, and many of that 20%+ could be non-symptomatic, but many of these newly-discovered myopathies have a later-in-life onset [typically between ages 8 and 15, and generally triggered by injury].
I've heard SO many stories from people where their wonder horse got cast at age 10 and basically was nutso after that, turns out the horse has P2 or P3 that was triggered by being cast - it's really hard for me to hear!
As science discovers more myopathies, it becomes less about if a horse has a myopathy or not and more about which one they have and if it's a myopathy you can live with.
The statistics are not good for me to be able to go out and get a "healthy" horse, and, even if the horse were free of muscular diseases, their legs could be junk, their mind could be junk, we might not get along...who knows.
Anyway, I still have a hard time with the idea of just letting disabled horses go without a second thought - "oh, you have a life-long illness? Here, have some euthanasia solution."
I understand the concept and I tend to agree in some circumstances [like, when a clearly disabled horse is in a kill pen and the homes that are lined up are ones that want to exhaust all resources to save one single horse that might not have a great quality of life in the end, or when a horse has been catastrophically injured and resources are exhausted to gain a low quality of life], but I also think that saying all disabled horses should be put to sleep might be an overly simplistic way to view the issue.
I think about this particular topic quite often, I'm very interested by it.