I think the only reason they haven't made a year-round TNBC ride is for the sheer fact that the limited time offer attracts people to the park. People like me. I am a huge fan of the movie and both times I have been to Disneyland in the Fall was so I could see the Haunted Mansion overlay and I ride it as often as I can. I understand not everyone likes it, but Disney is at the point where they use it as a marketing tool to attract as many visitors at a certain time of year. Same goes for Ghost Galaxy. I remember hearing a rumor around 2008 that they were going to end Haunted Mansion Holiday, but the way they're banking on the Halloween parties I don't see that happening anytime soon.
I think taking the space for the Winnie the Pooh ride and making it a TNBC ride would be awesome, but then you run into theme issues. Nightmare doesn't belong in Critter Country. I would love a year-round TNBC ride, but it would probably get shoved inside DCA somewhere.