Spirit Tracks sold about on par with the average Zelda game.
But I do believe like you it was delayed, but not because of Spirit Tracks not being well received but primarily because they were trying to work the controls to feel more natural. Nintendo knows it can possibly win the hardcore crowd with this Zelda, but they also don't want to alienate the casual and sometimes nostalgia fueled audience that has been built up on the Wii.
Right now, a scenario writer is probably crapping himself trying to tie this game into Ocarina of Time, along with a bunch of other older Zelda games, while still making it original. I can only hope that it is closer to Spirit Tracks in originality then other console Zeldas have been since Wind Waker. Can't keep ripping off Ocarina of Time and expecting it to sit with the Zelda faithful forever.
