I agree, if anything causes it it's most likely improper care, inbreeding and food that wasn't meant for animals that their owners give them and other such stuff.
My dog died because he was old, we did give him some left overs sometimes, but only of meat such as chicken after thoroughly washing it.
Honestly, if you want a healthy dog, get a mixed breed, the more mixed the healthier, give it the movement it needs and give it natural foods.
Pet food ain't bad for them but I doubt it is as good for them as we believe. Sure extra vitamins are added and stuff, but that does not make the majority of the content better.
It's just like our food to be honest, the majority of what we eat does not belong in a persons stomac. Diary products for example, we can consume them, but they aren't well for us, we just used them to replace bones for the way smaller amounts of calcium. I know a few african people who eat chicken bones and they got the healthiest strongest teeth I have ever seen. Same goes for dogs, if you give them bones regularly their teeth are better then that of most dentists. What you do need to watch out for is that dogs who don't eat slowly might attempt to swallow some bones whole. It's best to try to avoid giving those dogs small bones. In any case, you should always teach your dogs not to be greedy eaters as inhaling their food isn't good for them either way. Also some dogs got issues with some types of bones as they can get stuck between their jaws, make a good estimate and make sure your dog isn't able to jam the bone between his jaws. If you think a bone might cause that, make it smaller. Chasing a dog who's freaking out cause their mouth is jammed open sucks x) getting it loose is even trickier. Personally, when I give a dog a bone, I stay nearby to watch in case something like that happens.
You will also want to allow your dog to eat plants when it tries to (just know your plants to ensure it isn't a poisonous one and make sure that none of it has been sprayed with some chemical). If anything is likely to cause cancer, its allowing your dog to eat something with a chemical sprayed onto or near it. If you are a pet owner, you should NEVER use any kind of chemical or spray to take care of garden pests or weeds.
I don't think worming meds are the problem, as you only give them if your dog ate something bad and has gotten worms. Maybe the anti flees stuff but it's not something that you slather onto them everyday, in comparison I don't think it is that harmful compared to other things we do to our dogs or let them do.
If I had to guess tho, its the use of chemicals and such on garden and foods and that is behind the increase in dog cancer. I also really think that it is more related to breed dogs then mixed dogs.