Some reasons for my skepticism:
1. That majority sure isn’t emerging very fast: I first heard this thesis in the late 1990s. Teixera and Judis wrote a book on it that came out in 2002, which means it was written in 2001, or earlier. How did 2010 even happen? Yes, low turnout, energized base. But the GOP didn’t just squeak out a few victories; they crushed. Those of us who remember the 2004-vintage talk of a "permanent Republican majority" will be cautious about overinterpreting election results.
2. Ethnic coalitions are inherently unstable. It used to be a sort of natural law that urban Catholics voted Democratic. Then Reagan won them in huge numbers. And–contra those who are saying that the GOP now has to move left–they didn’t win by getting more liberal. Rather, the Democrats got more liberal, on crime and bussing, and the white ethnics who felt victimized by these policies fled. The more ethnic groups you have, the more likely it is that you will eventually find the goals of those ethnic groups in direct conflict. And the Democrats sure do have a lot of groups.
3. We are heading for a showdown between public sector unions and taxpayers. That’s going to put Democrats in a very tough spot. Those unions are the backbone of the Democratic political operation. But their pensions are, in many places, simply not payable. Thanks in part to the late 1990s stock market boom, and in part to really scandalously bad accounting standards, politicians made a lot of promises they didn’t pay for. Those promises now can’t be shed in bankruptcy, and all of the possible deals–which including hiking taxes to "tax revolt" levels, or shafting all the younger public sector workers–are bad for Democrats.