I agree that it is important to take care of your animals, but $5,000?
If all it took was money for my dog to continue living a good life, I would definitely pay it.
When it comes to paying to extend what is no longer a happy life, then I would put the pet down.
But I would NEVER put down or get rid of a dog for something as fixable as an ACL.
Yeah, I'm pretty much with you there. My black lab suffered a complete ACL tear just after turning 5. He was otherwise healthy and I had raised him nearly from 6 weeks to be my only companion through some of the hardest times of my life. I agreed to a TPLO, estimated at $4k but fortunately not much over $3k in the end.
Four years later, he's turning 9, chasing the fuck out of squirrels, and generally acting like a puppy. He ran/walked at least 7 miles with me on Saturday without so much as a limp afterward. Would I do it again? Yep. I'd get a payday loan for that shit. There's money, and then there's love. <3 puppies.
When you consider that a lifetime of proper care and feeding can easily run $10-20k, something like this is easy to justify, as long as financial ruin doesn't result. The real problem is people not accounting for those lifetime costs and then giving up on their pets when life surprises them.
All that said... if he gets cancer when he's 10 or 12, the calculus is a little different and we may just make him comfortable and happy till the end. It's all about quality of life, and if it can't be helped, we'll just focus on minimal suffering.