Absolutely, I agree with you 100% that you can't guarantee everything.
However, DM is one thing you absolutely
can guarantee. It is genetic, so if you don't breed affected dogs, you don't get DM. And it is only a $65 swab you send in and get results pretty quickly. There is absolutely no excuse a breeder, especially a GSD breeder, wouldn't do this test on every dog they breed.
DM was originally thought to be only in GSDs but now has been seen in other breeds. There are 3 types, clear, carrier, and at risk. Clear means two normal alleles, carrier has one affected allele but will not show effects of the disease, and at risk means they have 2 affected alleles and will most likely get the disease as they get older. It has nothing to do with hip dysplasia and OFA results.
It is a relatively new test, so your GSDs' parents might not have been tested back then which is why your dogs got DM. But now, it is an absolute must to ask a breeder the results or look up the results in the online database to see the results before you purchase a GSD or any other breed that carries the disease. It takes both responsible breeders
and responsible purchasers to eradicate this devastating disease from bloodlines.