Depends on his contract, I suppose. I think Oliver isn't super happy with the current HBO arrangement. I've read he'd prefer a block longer than 30minutes and more often than once a week. It's hard to be topical when you have 30minutes to talk about everything that happened that week.
His show is good though. It's just him ranting like when he was filling in for Stewart.
I watch Stewart and Maher faithfully. They are liberal, but they're not the voice of the Democratic party. Those comedians have far more backbone :P MSNBC is as ridiculous as FOX, by the way.
NBC, I think, offered Stewart to head their Sunday morning political circuit show. He declined. He does not like people considering him a reporter or having people use his satire as their sole source of news. He is a comedian first and foremost. I think he's leaving because there doesn't seem to be a place for him to proceed.
If you're working in late night and you grew up idolizing those hosts and watching those shows, your goal is to one day head The Tonight Show or The Late Show. Stewart, despite many multiple Emmy awards, being immensely talented, effectively making Comedy Central relevant at all, launching several stars' careers through his show working as "correspondents", and generally being a success - he's stuck on basic cable. Colbert, who he birthed essentially, got the coveted the Late Show gig. Fallon on the Tonight Show isn't going anywhere. All three of them are good friends. Oliver moved onto HBO which is a step up. So, Stewart sees the writing on the wall and is cashing out and moving on to spend time with his family and maybe direct a few more movies.
The Daily Show isn't going anywhere though. Stewart isn't the first host of it
