Hey, it's government work. Where do astronauts go when the active missions end? They can go back to what they were doing before they donned the space suit. The military astronauts return to what they were doing in the military. The civilian crew can hang around Mission Control doing experiments or giving talks about NASA and the space program. Or they can go into the private sector, working for a rocket-development company, perhaps.
Truth be told, the number of NASA astronauts has been shrinking; in 2000, Houston had 149 astronauts; now, there are around 60. Its last group of recruits, which got there in 2009, had 20, all mission specialists.
The trouble is astronauts want to go into space. But since the demise of the shuttle program, NASA doesn't need any pilots, and there's a slim chance that astronauts will get to the space station. So there has been a brain drain, with 20 leaving the program in 2011. NASA does have plans for its space program. It hopes to develop a space capsule that can ferry four astronauts into deep space to places like the moon, an asteroid or Mars. The goal is to have such a capsule heading off to space by about 2016. NASA will need astronauts for that. But now, it's just wait-and-see for the astronaut crew.