Unfortunately, I wouldn't be able to justify a lot for our dogs. We have three.
If it was something like a broken leg - a simple procedure, localized... that's one thing. But if one of our dogs got hit by a car, got a terminal illness, that sort of stuff... they'd be put down.
We have a big 'farm' mentality here... Most of the medical work for our animals is either done by us or, well, not done.
And i'm in school to be a vet tech, so this isn't a "I hate animals" type thing. It's just how our family works. I can't justify going into debt or paying savings-account money for an animal, when the next day DH/DD/D? might break a leg and have to be cared for.
As harsh as it might sound, animals can be replaced. There are plenty of shelter and rescue animals who can benefit from having a family... and your "middle-aged dog" has already had a great life.