This seems to be a movie construct: there's no evidence for inbreeding, nor any for lack of inbreeding, in the books. The most that is said is the quote from the Silmarillion regarding how Orcs reproduce:
For the Orcs had life and multiplied after the manner of the Children of Ilúvatar...
"Uruk" is just Black Speech for "Orc" (derived from a primitive Elvish form) and doesn't necessarily need to denote any special breed of them, although in practical use it came to do so. This is referred to in Appendix C to Quendi and Eldar, published in HoME11:
For these shapes and the terror that they inspired the element chiefly used in the ancient tongue of the Elves appears to have been *RUKU ..... except in tales of the ancient days and the March, and then is vague in meaning, referring to anything that caused fear to the Elves, any dubious shape or shadow, or prowling creature. In Sindarin urug has a similar use. It might indeed be translated 'bogey' ..... The Orcs themselves adopted it, for the fact that it referred to terror and detestation delighted them. The word uruk that occurs in the Black Speech, devised (it is said) by Sauron to serve as a lingua franca for his subjects, was probably borrowed by him from the Elvish tongues of earlier times. It referred, however, specially to the trained and disciplined Orcs of the regiments of Mordor. Lesser breeds seem to have been called snaga.
This is the same root seen in Quenya "Valaraukar", Sindarin Balrog, and as such is a quite generic term.
"Uruk-hai" means "Orc folk" and therefore any random Orc passer-by, when speaking in Black Speech, might refer to himself as a "Uruk" and his family as "Uruk-hai". But like I said (and like Tolkien wrote) "Uruk-hai" eventually came to refer to the more superior breeds, in what I find to be quite an interesting parallel between "Men" and "Hobbits"; i.e the generic name became restricted to the larger kinds with the smaller kind picking up a separate name of their own - which for Orcs was "snaga" - let's not introduce "goblins" here, please!
That was quite a diversion, but the purpose is to demonstrate just how much this concept had been changed between book and movie. In the books "Uruk-hai" are certainly not a decaying, inbred subtype; they're actually the superior subtype.
