According to a report on WENN.com (reported on IMDB), Bay feels that with forthcoming sequel Revenge of the Fallen, he has taken the series as far as he can.
"After the three-and-a-half years I've spent making these movies, I feel like I've had enough of the Transformers world," he explained. "I need to do something totally divergent, something without any explosions."
http://uk.movies.ign.com/articles/996/996050p1.html
And in a sly dig at journalists, Bay added: "It's easy to go shoot an art movie in a winery in the South of France. But people have no idea how hard it is to create something like Transformers. They [the critics] review me before they've even seen the movie."
I don't think anyone is denying there is a certain degree of work to be put into a movie such as Transformers, but that is not at all a scale on which to rate it. It needs more of an intellectual push to make something without explosions. Are you sure you're cut out for it?