Society is always changing and evolving. If you want to call that "becoming more liberal", so be it (although it would be a simplified description of the phenomenon).
A reversal of such societal changes is pretty unlikely. For instance, there won't be a rollback on gay marriage/civil unions, just like you shouldn't expect racial segregation to be re-introduced or women's suffrage to be repealed at any point in the future. In a sense, conservative movements were never able to prevent social change, they only succeeded in slowing it down.
Of course, all of this hasn't much to do with a "GOP comeback". The Republicans will hold the more conservative position on political issues for the foreseeble future and they're also going to win elections. The definition of "conservative" can and will change over time though.
What I mean is that what your saying won't happen could happen, because there is so much time, but when? I personally would rather live in a 19th century world with the exception of the lack of medicine. Sooner or later the new thing will be Conservatism. The sun won't blow up for a really long time you know. That Democrat from Louisiana is right. The world is always changing, and for now it's just getting more and more Liberal. Eventually they will get so Liberal that they can't get any more Liberal, so everyone will start getting more Conservative. Of course it's hard to tell what will happen in a million years but you never know. Something else I predict in the pretty near future is that I'll be banned from the forum for being too radically Conservative.