I'm sure QC fans wouldn't be the only demographic. The question would be whether or not they were the primary demographic. Comic book movies come to mind when I think about this, 'cause many of them were perfectly good movies that grossed well, but comic book fans hated them (of course they went and saw them anyway, as I'm sure most of us would for a QC movie, even if it got terrible reviews by other QC lovers). A movie ending with Marten and Dora together would probably be just fine, though. To the fans of the comic, it would just be "part one" so to speak, even if there were no sequels. To people who had never heard of the comic, it would be a good happy ending.