While I don't like the thought of our horses being "disposable" and certainly don't follow that line of thought, my previous horse DID have chronic lameness issues and I ended up making the decision to donate him to a therapeutic riding center.
After he tore two massive holes into his RH suspensory (two different branches of it) he was never the same. He had 1.5 years off, with 6 months stall rest and 1 year turnout. We slowly brought him back into work and he seemed to be going pretty well, minus some stiffness that a joint supplement took care of. Before I put him up for sale as a beginner horse I had a pre-sale vet exam done, and he flexed 4/5 on that same RH. There was no point in doing ultrasounds at that point. For whatever reason, all the shockwave, stall rest, cold hosing, etc. in the world hasn't helped. He was a lesson horse for about 8 months, doing mostly W/T for beginners, and after 3 months off for the winter while the owner went to South Carolina, he was almost dead lame at the canter, definitely off at the trot, and just short behind in the walk, even out in the field.
If I have a horse with chronic issues, I have no problem finding it a better place and buying a new horse. End of story. I can't shovel money into a horse, no matter how much I love it, for continuous, serious lameness issues. I can't. It's not feasible. I'd rather get them healed up as much as possible and let them go on to something that won't tax them anymore than they can handle and then put money towards a horse that I can more reliably count on to be sound.
If I had the money to buy a horse after mine went lame, I probably would. I'm not ashamed to admit that in the slightest.
"Always be yourself. Unless you can be a unicorn. Then, always be a unicorn."