Right, but... Rathalos isn't just an assist trophy, but a full-on boss. In my eyes, that's a significant step up in importance, considering how few bosses there are. It's hard to know exactly how important/development-intensive different types of content are, but I'd guess a boss is more comparable to a stage than just an assist trophy. If we got Ancestral Steppe as a stage and Rathalos was a stage boss, would we still be talking about Monster Hunter? I mean, technically, that would be the exact same amount of work, because Rathalos does have a unique boss stage.
Also, I don't really see why it not physically being a Monster Hunter matters that much. MH content is MH content.