Anecdotally, I always felt like 'muts' were some of the healthiest dogs I've been around. They seem invincible, and the genetic diversity probably helps explain that.
Out-crossing a purebred with something from a different style of dog can often produce much healthier pups than either parent. Great Danes and Collie make 3/4 sized, slightly skinny dane-like dogs that are shockingly robust and energetic and can live to 16+ in my experience (and that's as active working outdoor dogs).
How would they fare as indoor, backyard dogs taken for twice daily sedate walks though?
My personal (probably unpopular) opinion is that a lot of inner city people should not have the dogs they have - who need to be able to run around at all times, not just waste away.
It's also why I don't have a dog, for exactly this reason - I don't live in the country, with enough free space around.
Interestingly, the notion of breed changed meanings, for me, when they published the results. The only modern breed in her is a great grandparent chihuahua and otherwise she’s genetically varied as feral dogs are, something like an asymptote for the various blends called mutts.
Of the 161 (i think) tracked recessive disorders, she’s a carrier for only two.