Just as Clinton's Third Way was only effective because of Clinton's political charms, I believe that Bush's evangelical strategy is only effective as long as he, or someone equally conservative and religious, is in office. Which means it could lose effectiveness as early as 2008.
Would evangelicals be as eager to embrace Arnold Schwarzenegger? Rudy Giuliani? John McCain? Bill Frist? What do these four have in common -- they don't wear their faith on their sleeve. Hell, Ah-nold probably doesn't even practice religion. And they have weird... progressive ideas. That doesn't mean that none of the four couldn't win, but none would be the evangelicals' darling like Bush; therefore, they might have to work harder to appeal to different groups of people, much like the Democrats do now. That could neutralize the advantage Republican presidential candidates typically have. Consider the ones who won since Nixon -- Reagan and Bush* (asterisk, because he didn't win the first time, and had he not been an incumbent, wouldn't have won the second time) were both openly religious and culture-driven. Bush I and Ford were not.