The show will continue as long as they can keep a handful of the regulars on board. It's the #1 scripted show among 18-54 year olds right now. This show is very similar to NCIS in that it's constantly gaining viewers somehow.
I think it's because like NCIS The Big Bang Theory is rerun constantly. It's on all the time on TBS and in local syndication. People have caught the reruns and have moved onto watching the new episodes weekly.