I don't necessarily think that cancer treatment is always in a person's best interests. It's painful and quality of life may be greatly reduced. As far as I'm concerned, individuals should always be able to refuse medical treatment instead of being forced to endure increasingly extreme medical measures. Preserving life "at all costs" including situations where we'd put down a pet that was suffering that much is a a fixation I don't agree with.
A child, of course, cannot decide about treatment, but I would still think it could well be better to let a child with extreme defects die rather than undergo multiple surgeries or other treatments. I'm not convinced childhood cancer is always worth curing, especially when it requires suffering through chemo with less than great odds of success.
Of course that isn't the grounds on which these breeders are challenging - the view of a natural death - but on the basis of being too stupid to understand the treatment. So they are idiots.
Of course, I would then take things a step further to question the morality of natalism in the first place, especially where it is known that an individual's suffering will be above average. Breeders like the ones in question probably won't think twice about the fact that their choice to reproduce led to this unneeded suffering and death.