I don't recall seeing anyone say "no on-foot, no buy," and, if they exist, those people are tremendously outnumbered by those saying "on-foot = no buy."
Can't speak for everyone, but it seems there are people who would like a vehicles-only, strictly on-rails Star Fox, but are content with existing SF games that follow that formula as there isn't that much room for improvement compared to the on-foot, vehicle switching gameplay of Assault. Even to many fans of the game, Assault is flawed, and those fans would like to see what it did right built upon in a new game and what it did wrong be corrected. A new SF just like 64 would be, by comparison, a rehash of a good, yet basic formula that didn't need any refinement. Such a game would be a missed opportunity for something potentially greater.
Oh damn... I respectfully disagree with you. There is so much that can be done with a new rail shooter that hasn't been done before. The entire genre has been fairly dormant for a decade.
I'm sure people felt the same way about 3D platformers before Super Mario Galaxy came out. That game was both familiar and totally fresh at the same time. Nintendo shown that they can have a new and unique take on old genres that totally recapture the imagination of their audience. I think there is plenty that they can do with a pure rail shooter that would be a far cry from a "missed opportunity".
In fact, they have far more opportunity to innovate in such a pursuit than they would in developing a 3rd person shooter game with vehicles.