It's difficult to use RE4 in the argument either way. It was $30 and it had all the content that Nintendo fans missed out on, not to mention Wii remote controls that made the game easier, but worked extremely well. On top of that, despite its age, RE4 is still, sadly enough, one of the best games on the console.
Still, I'd argue that there is an audience for these so-called hardcore games. Many people who bought RE4 on Gamecube, including myself, wanted it for the new content. If Capcom made a brand new canon title with the same gameplay engine, they're automatically appealing to everyone who bought the Wii port, Wii owners who didn't buy the port but have/played it on Gamecube, and finally fans of the series who would buy a Wii for some of that sweet, sweet RE lovin'. Without using sales of a good brand new title as proof, assumptions about the Wii userbase are unfair because there are many more indications that point to the presence of an audience for these kinds of titles than there are the other way around.
Since Wii's large userbase isn't enticing enough, Nintendo only has one card up its sleeve: cheap and easy development. I think they really need to start selling this aspect to 3rd parties. Although brand new titles would be better, even a remake of an old, popular game could help Nintendo at this point. Something like a FFVII remake would sell regardless of platform, but it would still prove that people who want that kind of game now already own the platform and are ready and willing to buy RPGs. Of course, it could backfire with publishers arguing that only FFVII could sell that well and knowing Nintendo's luck, that's probably what would happen.
Yeah, I think Nintendo is kind of screwed. No matter they do, there is just no winning.