Yeah, Secret Invasion felt like the "end" of his run, then Siege happened (which was actually pretty good) and then that felt like the end. Then it just kept going. The beginning of The Heroic Age would've been the perfect time for him to exit. After "The Avengers" came back and "New Avengers" relaunched, I just kind of lost interest.
I like Bendis just fine, but I think, much like Geoff Johns, he enjoyed "sticking the landing" so much that he tried to stretch it out as far as it'd go, which can get tiresome.
That's because he really didn't plan ahead. I guess we'll never know if it was his fault, or if it was just impossible to accommodate Marvel's editors mandates.
Hickman ended his FF run with a bang, Bendis ended his Avengers run with a whimper...that's the difference planning ahead makes.