I have NO PROBLEM with creating new breeds - American Hairless Terrier, Alaskan Klee Klai, Miniature American Shepherd, White Swiss Shepherd, Biewer Terrier, etc. If you have a new niche or job for dogs, then I am in all in favor of you working with other breeders and developing a true-breeding, distinct population of animals that meet your needs. Go for it!
However, if you are talking about the "yorkipomalabraboxabullajuggadoodledoos" then I have a problem precisely because these *are not new breeds* they are not defined by traits but by BEING MONGRELS of the same breeds, they do not "breed true", and they are unpredictable. Because they are unpredictable - which 70% of this litter of "Goldendoodles" will shed? You can't place them in appropriate homes and many will end up being homed and rehomed and dumped and dead in shelters. 50-70% of Poodle mixes shed, most Pug mixes have breathing problems, temperaments are all over the board as are physical types to the point where many dogs have just the "wrong" combination and are even unsound.
I don't have a problem with owning mutts, but I have a problem with breeding ANY dogs when you KNOW (or should know) that you will be producing many dogs that have less than decent odds at a great life - dogs with the spindly legs of the Poodle and heavier body of the Schnauzer that will have arthritis by age 5, dogs with the profusely shedding undercoat of the Golden and the tight curl of the Poodle that will end up with constant pain from hot-spots and matting, dogs with the energy and drive of the Beagle to go-go-go and the breathing apparatus of the sedentary Pug that will end up with heat stroke repeatedly.
PLEASE go make an actual new breed! But please don't call it a "breed" until/unless it actually is one. And please go about breed development ethically - not selling first (or fifth!) generation mongrels as something "rare" and "special" with some perfect combination of magical traits at inflated prices to the uneducated public. Place your "pet qualities" carefully and be willing to take them back for life, and please don't breed at all unless you are actually confident you are making progress toward litters where more and more pups have better and better odds of predictability, health, etc.
ADD: The UKC used to be reputable, but it is rather common knowledge that their bully-breed registering is suspect at best - falsified pedigrees of dogs that are clearly most something other than the purported breed, etc. The "American Bully" I have issue with because they can't walk. Even the English Bulldog has a better claim to soundness! Also, they are supposedly all derived from "purebred APBTs" or some such happy fiction, when it is rather clear that there is Bulldog and a few kinds of Mastiff in there - it bugs me that the breed are mutants and that they are based on lies! As for the Utonagan, my impression from researching the Tamaskan and related breeds makes me have a problem with their breed "club" and breeders, who all appear to be dysfunctional drama queens who left the Tamaskan folks in a huff when the latter worked to impose some basic ethical breeding guidelines - they have several small, clique-ish breed "societies" and "clubs" that don't talk to each other or agree on anything! I would love to see that breed come to fruition, but I can't find any evidence it is moving toward a standard or a stud book or seeking recognition anywhere.
ADD: Please give the citation for your statement: "scientifically speaking it only takes four generations of consistent breeding to create a purebreed [sic]". Having bred poultry (ducks) I can tell you that it often takes more than 4 generations of breeding within a breed, with no crosses, to just get COLOR genetics "set" and breeding true. There are still Labradors born with tan points and/or brindling even though those colors came from crosses 100-200 years ago and have been bred-out as much as possible since!