Similarly, I think the official breeding systems for purebred dogs are damaging and fundamentally flawed - even much of the testing done and the restrictions on breeding "pet" dogs - because it allows people to think that issues in the breed are really being addressed, when in fact the main effect is to further restrict the genetic variation in the population which long term will be the destruction of many breeds and actually locks in problems.
I think this is a rather simplistic view of a complex problem and is, in part, likely informed by thinking about dogs like Frenchies (for example).
There are definitely issues with pedigree breeding. But there are also a lot of responsible breeders out there, including myself, who breed selectively to ensure healthy robust puppies that are capable of doing the work their breed was bred to do.
'Genetic variation' can more than easily be done within a breed without resorting to mixing breeds to get, what ultimately, are very expensive mongrels - which, I assume, is what you mean. That mongrels are healthier than pedigrees.
This is very often not the case. Healthy, robust, pedigrees with the right temperament and health tests - that everyone should want their dogs parents and grandparents to have had - are never bred with other breeds to produce these designer doodles or mongrels. There is a reason why so many of these designer doodles do have so many behavioural issues - a combination of poor breeding & poor ownership.
My dogs, for example, are some of the healthiest (in terms of health scores) that I know of for their breed. But I would never breed my girls with a Poodle to produce a designer doodle. Why? Because I breed to make my breed better.
The sorts of dogs that are used to create these 'healthier' designer doodles, are very often not the best examples of their beed. Therefore the myth that 'mongrels' or 'doodles' are healthier is actually wrong, as very often their dam/sire are not the healthiest dogs (hence why they are not being used for pedigree breeding).
The problem is, too many people buy into false narratives online (gosh, where have we seen disinformation before...) and are also so demanding about wanting 'this dog or that dog' or wanting a dog 'with a flat face because it's cute.' Instead of prioritising, healthy, happy dogs from good breeders - which was OP's point.