Don't feel like a failure.
If this were someone else asking for advice, you know you would say that there is only so much you can do to make up for bad genetics.
What would I do in this case?
I would take myself and my emotions out of the equation, and look at this only from the perspective of the quality of life of these animals, and what the future holds for them.
Are they happy? Are they thriving? Are they pain free? Are they mentally in a good place?
Then I would make my decisions. And I would make those decisions for them, not for myself.