No, the threads do stay- to see the older ones you just have to change the time limit at the bottom of the page. If you look at the older series like I'd Do Anything and Any Dream Will Do, you'll see that a) most of the contestants' threads are still used from time to time, even if it's only once in 6 months, and b) you can actually still read the threads that people posted for the very first show!
By the way Eleven_Eleven, thank you for all the news updates you've given us. Also really don't worry that because people aren't posting on here it means that they've forgotten Lauren. This thread never was as Twitter-like as some of the others and as a result, I suspect a lot of people who've contributed in the past just won't bother posting if they don't have anything in particular to say (I include myself in this category). Obviously I can't speak for anyone else but I still check this thread at least once a week in case there's any news or any clips/ interviews.