Matt developed the show with Sam Simon and James L. Brooks. There's a team of ten writers in house, and the head writers now are Bill Oakley and Josh Weinstein. They are also the Executive Producers, also known as the Showrunners. The writers change occasionally, which is pretty much the nature of most comedy shows. I think what happens is that people burn out.
People can burn out even if it's a great show.
They're looking for submissions for possible replacements in the future. Some people got on staff because they knew somebody. If you don't know somebody you have to be very good. There's no lack of story ideas. There's a list of ideas that have yet to be realized, so there's never a lack of material.