Bay knows his limitations. He can do great CGI and non-CGI action sequences that look like a million bucks (usually because they're more than that
), he can objectify attractive women and he can do low brow comedy. The one time he tried to leave his box, Pearl Harbor, ended with bad reviews and an underperforming box office after OW. He has been wise to stay in it since.
Now, many comic book films have shot for what Bay does and missed the mark because their directors lack his vision for action (Brett Ratner and Tim Story come to mind). However, I think Marvel has played it smart by getting outside-the-box directors for the most part. Favreau, Branagh, Whedon and now Black. Get directors who can make great movies without big budgets or CGI. Then see what they can do with that extra muscle. Nolan, Raimi and Singer also fall under this category.
Lastly, only a few directors can sell a movie. Spielberg, Cameron, Scorsese (for his audience), Tarantino (for his audience) are the only ones who almost always guarantee that right now. Bay casts big movie stars or big brand names in his movies (Will Smith, Martin Lawrence, Bruce Willis, Ben Affleck, Liv Tyler, Nicholas Cage, Sean Connery, Transformers, etc.). The one time he did it without movie star brand names (though it had well known actors) was The Island and that flopped. It's unclear whether his name sells a lot of tickets or not.
