Every Star Fox since 64 has sold badly, and every Star Fox since 64 has been very different from both of the first two titles. They just plain haven't given us another true Star Fox game since then, so we really DON'T know how one might sell.
That said, I wouldn't be surprised if this one does well, but I wouldn't be surprised if it floundered either. 64 came at a time when that sort of thing was new and amazing. Now if they give us a 45-minute on-rails shooter and try to sell it to us for sixty bucks they're probably going to be disappointed. A new Star Fox needs to be huge and have a lot of content and spectacle and multiplayer that blows us away. Aces of the Galaxy is basically everything you could expect from a "modern" Star Fox, and you can get that for only ten bucks, so Nintendo's got to do a lot to get my money.
EDIT: Apparently Adventures and Assault didn't sell super terribly, but there's definitely been a decline. Point stands! *pounds table*
Currently playing:
Bloodbath Paddy Wagon Ultra 9
