I agree that their quality has gone down hill since the release of Fable 2. I think the release of the third game was too soon and there wasn't really anything new other than story. I feel like this franchise is starting to look like Assassin's Creed where it's the same mechanics but new story.
I'm no game developer and have no idea the time and hard work that goes into it, but I feel the franchise is lacking. Just my opinion.