They could have waited till they get more republicans in a month.
What good would that have done? The Republicans don't have a majority in the Senate, so any law would have required brokering a deal with the Ds.
Moreover, the reason why these tax cuts had sunset provisions in the first place is that the Rs didn't have the votes necessary in the Senate to make the cuts permanent, and that was back when the GOP controlled 57 seats or whatever it was.
A deal was going to be made at some point. As far as deals go, this doesn't seem like a terrible one, and it's a blow to Obama's base. More troubling from the GOP perspective is this: does this deal signal Obama's shift to the center, ala Clinton in 94?