I think the only way they can bring Rob back and pay him the money he wants is after Martin Sheen is off the show. One of the reasons that Sorkin was let go (in addition to not turning in scripts on time) was that he was VERY spendy when it came to the show, and the network wanted them to cut back drastically.
You're the only person I've heard who said that the writing has improved under John Wells. I hate to admit it, but in some respects I kind of agree. The last two seasons have been the first time I've been able to watch the show without subtitles because people speak more clearly and distinctly now. They're not trying to jam in all that Sorkin-esque dialogue in 15 seconds.
BTW - Sorkin has said that he wants to write the final episode of the show, whenever it airs. He's also said that if he writes that episode, he wants Rob Lowe involved somehow. The ratings for the premiere were terrible, so I'm thinking that this is the last season for TWW, and that's yet another point to my spec that we're going to see a McGarry/Seaborn ticket (or Seaborn/McGarry) before the season is out!