a dog that will have very little quality of life at the end of it because a double hip replacement, double knee replacement and the depression that months of confinement and limited mobility aren't easy to recover from
What? Luxating Patella and Hip Dysplasia surgery have very high rates of success. Particularly in a young, otherwise healthy dog. It isn't 'months' of confinement either. And you've probably caused the hip issues by not getting the LP sorted long before now. You haven't even done X rays so I've no idea why you're continuously implying you've exhausted all avenues.
As for And honestly I don't know where all these magic rescues with unlimited funding for medical treatments are
You've been told, more than once, SpanielAid, who literally state the following on their surrendering page:
"My dog has a medical issue, will you still take them?"
"Yes, we will never make the decision on whether or not to accept a dog into our care based on the cost of their care. However, we will need to ensure that we have a foster home suitable to look after your dog, and this may take a little longer if the dog will need ongoing care".
Utterly appalling. I don't know who you think people on The Dog House are exactly, but there are plenty of long time rescuers and ethically sound breeders here, you seem to think we must be as green as we're cabbage-like. If you want to put a young dog to sleep, who has a decent chance of a good life via rescue, then do it, but stop attempting to salve your conscientious by repeating the excuses in the hope someone agrees with you.