
Well, I saw my dog rupture her cruciate after she caught her leg in wire mesh as she jumped over a fence and I would like to see any dog come through that unscathed! She had 12 pups, none of which have done similar and as my vet says, the more free hard exercise you give your dog the more prone it is to injury. My dog died last year aged almost 14 & half and never had a problem with her joint after her operation, not even slight lameness.
Likewise any one competing in a physically demanding sport is more prone to injury. Was Paul Gascoigne excluded from having kids when he did his knee in because that was exactly the same thing as a ruptured cruciate in dogs?
As for cancer it would depend on what and where. I have friends with a Flatcoat who had to have a toe amputated due to cancer a couple of years ago, he's now 10 and was given the all clear ages ago. Likewise friends with a Munster who had to have his tail amputated due to cancer 4 years ago. Last month he celebrated his 13th birthday!
It is too easy to generalise without looking at each case individually.