Beef11 - It is obvious from this post that you have given this subject a lot of thought and consideration, and I applaud you for your perspicaciousness (How is THAT for a descriptive word?? , and , unfortunately, I can easily understand how your scenario could be in effect. The answer, in my opinion, to negating the "mongrelizing" possibilities is the same protocol that all breeders should use in ANY breeding program - ELEVATING Genotype and Phenotype traits and characteristics in ALL seedstock whether purebred OR crossbred! I am sure that every breeder who has been in the business for any period of time at all has seen some crossbred bulls and cows that were better representatives of fine breeding stock than a lot of purebreds - and those are the kind of crossbred bulls and cows I would be seeking for a Terminal program, or if I were raising stock for a Terminal seedstock market.
In a lot of the research papers that I have on crossbreeding beef cattle it seems that even the researchers tend to stress the COLOR that results from crossbreeding! That is just plain HOG (if you'll excuse the term) WASH! Optimal EPD's and Phenotype will circumvent "color" if the IMF, uREA, uFAT, Yield Grade and $Profit factors bring the producer money in his pocket for the effort and $$ expended. He is NOT raising "sweet" and "cute" and "adorable" in spite of the fact that they may be all of those things. Longhorn breeders may be the exception to this rule, but for the most part they are not raising "WHAT'S FOR DINNER!"
Crossbreeding is a heck of a lot MORE than just a niche!