I think that is likely exactly the reasoning behind it. We know he was not approached with the idea that he would take over Superman. He asked for the Superman books specifically and was given complete carte blanche to do with them as he sees fit (I just listened to the NYCC Superman panel with him and Frank Miller and Didio at one point says that editorial's plans for one character were to drop them out of the book for a while, but Bendis changed that). It's not like they brought him in thinking they needed him on Superman to boost sales. He likely has these books until he tires of them himself or tells the story he wants to tell and moves on barring a complete collapse of the books sales.
And regardless of his selling power, he does have a lot of organizational pedigree over the events and arcs of Marvel books across the line. They likely factored that in as well, with John's moving over Bendis along with Snyder seem to be the ones guiding the overall narrative of the DCU into the next few years. I don't get the sense that King, big name that he now is for them, has the interest in taking on that kind of role. The Wonder Comics imprint is a good example of that. He's spearheading and guiding a whole new line of books that fit right within their publishing plans.
So yeah, while I'm sure better sales on the Superman line would have been welcomed. I don't think Superman and Action make or break his deal with DC.