Just to play devil’s advocate: from a genetic standpoint, wouldn’t mixed breeds be more robust anyway (i.e. less prone to disease and disability)?
Every breed has genetic/inherited health issues and mixed breeds will still express those health issues just like purebreds will.
In some cases, combining the “wrong” breeds can even increase health issues exponentially, such as mixing two breeds who are predisposed to hip dysplasia or osteosarcoma, as examples.
With a purebred, you can make a reasonable guess as to which health problems you *may* face during the dog's life and it's easier to diagnose illnesses in your dog by deducing what it's likely to be afflicted by, according to breed statistics.
With mutts of unknown parentage, it's a crap-shoot.
That is the whole point of having a “pure bred dog” in the first place; to set type/temperament and homogeneity so you know what you're getting, as much as humanly possible.
Even the word “hybrid” is a misnomer as a mixed breed dog is still just a dog and not technically a “hybrid” of anything.
A mule would be a better example of a hybrid.
I dearly wish "hybrid vigor" ~were~ a real thing because I'd be busy creating the perfect Dobe mix that was disgustingly healthy and obscenely long-lived...:-\