OK - I'm a huge fan of normals, the normals in my collection are some of my favorites. But your theory makes no sense. If I have a mojave male from Breeder X and a mojave female from Breeder Y, I've just outcrossed the bloodline. Neither mojave shares common parentage.
If I breed a pinstripe to a black pastel, I've just outcrossed the bloodline.
Why would you assume that those breeding homozygous animals to homozygous aren't acquiring animals from different and diversified bloodlines?
I would have to go look, but I believe someone once shared the statistic that cornsnakes have been line bred for up to 10 generations with no marked increase in genetic defects.
Genetic defects like kinking in caramels or wobbling in spiders appear to be linked to certain morphs, not from line breeding.