I think they really had already gone as far with Gardner as they could. The show is supposed to be about Alicia, and the back and forth between L/G and F/A was getting a little boring. Without a good reason for Gardner to be in Alicia's orbit, I suspect the plan for Gardner's character was going to be too lightweight for Josh Charles' liking.

By the same token, I don't understand why Christine Baranski is still there. My prediction is that there is going to be some reason for her to stay, i.e., some reason for Diane to remain in Alicia's orbit, including, but not limited to, a incorporation of F/A into L/G, or some other shuffling the deck, i.e., the break-up of L/G bringing Diane to F/A (or, more likely, L/F/A). Just a guess.