With that stipulated:
If the benefit a dog obtains from living outweighs --- in all but the most extreme cases --- the suffering that dog will have due to its genetic defects, AND people are entitled to make a rational decision not own a dog, are they not ethically entitled to select any breed they want? How would it be more ethical to create no benefits for any dogs than to create compromised but net-positive benefit for some dogs?
a) Given existence: significant suffering, but not enough to justify ending the existence
b) Given no existence: creating an existence knowing you'll create the same amount of suffering as in (a)
Something meaningful is created by the very fact of existence. Or, should I say, already existing matters a lot.
Were no dog available that could reliably provide those attributes, our choice would certainly have been not to acquire a second dog.
But I also think, for what it's worth, that it's still ethically tricky to dispose of the question of whether it's unethical to select dogs for appearance. I think you can predict the argument I'd make there.
I'd say it's probably unethical to breed more brachycephalic dogs, and by extension unethical to purchase those dogs from breeders (unless maybe the breeders have ceased to breed these dogs).