But, big but, and jumping in without reading the thread....
Which breeders will allow their dogs to he used to be bred in mixed breed pairings?
In my experience, hardcore breed advocates generally produce the best of their breed. They understand the lines, the conformation, the genetics, the dog as a whole, the working traits/temperament etc etc etc. And they are ALL ABOUT preserving those qualities.
I don't know a single breeder who would sell a puppy to someone intending to use a very well bred dog from excellent lines to produce a companion mixed breed.
Which leaves the door open to who exactly? People purchasing dogs from people who maybe don't care so much? Or people dishonestly purchasing dogs to use without the breeder consent or knowledge?
I don't have any issue what so EVER with WHAT is being bred, but it's how it's done, and I personally don't believe you will get the best of the best, and we should only be breeding the best of the best out of love for our dogs - to breed diverse, healthy, correct animals without any issues. And I don't think that will happen with mixed breeders for the above reasons.
I'd happily buy a mixed breed if I thought as much passion went into it, and the dogs were top notch. I don't have issue with the outcome of mixed breeds, I will probably own more in the future

but oops, rescues, rehomes.
And 90% of dogs from the best breeders will still end up being companions.... Not every dog in every litter is going to be a top working or show dog. Even some entire litters don't make the cut!
I don't disagree with mixed breeds at all, but I just don't see how it could ever ever be done up to true standards people will take to maintain any single breed. So for that reason, I'm out!