Well, isn't her contract up this season? She has said since the end of season 1 that she did not intend to renew her contract, despite where DH might be at the time. That it was something she enjoyed doing, but didn't want to do forever, and that she was happy with the length of her contract. She's alread done a couple movies and has great agents that promote her well.
And S y is right about Felicity Huffman's character having cancer (and there is someone who definitely doesn't need the show to launch projects).