IMO, a dog is a dog. My mutt from the woods is the best dog ever, but I also love our GSD and lab. Dogs can have different jobs and some dogs and some living situations don't mix, but a dog is still just a dog, whether it was free or cost you 2k.
Yeah, poorly bred dogs can be prone to behavior issues and physical issues. Also some dogs just have problems. I would not want a dog with severe issues because it would just be pain for the both of us. In the end it is kinder to put a dog down than to let it suffer. However, if I had a dog with issues, I would not let it be in pain. It all depends on how long they're able to live comfortably. My grandparents just had one of their minis unexpectedly colic. After fighting it for almost 2 days, the horse was in incredible pain and still impacted. They had to shoot her.
There are people out there who just dump dogs who are sick or old. That's terrible. It's not the dog's fault that it's old or hurt or sick.
There's that huge debate about the pitbulls and whether they're safe or not. My dog probably has some pit in her but we're not sure. She was slightly aggressive when we got her, but with training she's the best dog we've ever had (she doesn't like kids at all though so when friends come over she usually goes up). I think pits that were bred for fighting can have that aggressive streak, which is what they were bred for. So, some of them are dangerous. Now nature is only part of what determines a dog's behavior, training is a lot of it. I think some pits are totally fine, but even so I'd be careful around them.
Unfortunately, others won't give proper care to an animal unless considerable money was invested in its purchase. Some people need an incentive to do the right things and sometimes that incentive is to be required to shell out serious money. People who pay $2,000 to $3,000 for a puppy won't likely starve it, fail to groom it or fail to give it fresh drinking water. This dog will likely be vetted with religious devotion and trained properly.
If the person's only motivation to take care of the animal is the fact that it cost them money, then it's debatable whether they should have gotten a dog or not. Sure, there's economical sense to take care of an investment, but that purebred Australian Shepherd feels pain and hunger the same as some no-account street dog.
Main point: It's people that are the problem. Irresponsible, cruel, lazy humans are the cause of a lot of problems with animals. Some people don't let a farrier see their horse because it's too expensive. Some people dump their old dog or cat because taking care of them is too expensive. Personally, I probably wouldn't get a purebred. Why? Because I'm broke and I'm not spending 2k on a darn dog. Now, if I was a person who hunted with labs I might pay a lot for a lab from proven hunting stock. If I wanted a sheepdog I'd get one from good herding stock. But just for a companion? Breed shouldn't matter so long as the animal is safe, happy, and comfortable.
It's like horses. You want a jumper that will carry you to the grand prix? You're looking to spend a buttload of cash. Just want a trail horse? Not nearly as much. But frankly, abuse happens to horses too, no matter what they cost. You could have a racehorse that won a lot of money be sent to slaughter if they were retired and useless for breeding. An animal cannot help having issues. In the end, it's people, not the critter.