What's with all this talk of in-breeding. No good to anyone and therefore no-one would do it. Whereas correct line breeding, with occasional (correct) outcross where appropriate is how you produce consistently good off spring.
Continued outcrossing is hit and miss. Hence a lot of what looks like a fantastic pedigree produces disappointing foals. Successful breeding is all about conformation and temperament maintained through generations and such consistency can only be achieved by careful line breeding.