I personally don't see it as a rip-off. Maybe it could have been downloadable content, but as stated the game is going to be making some pretty big changes. It's on a disk and it won't be full price. The fact that it isn't full price, I'm very happy with, though I would've invested in it anyway. 8 new characters and new moves is a huge upgrade for a fighting game. I've already got a lot of mileage out of SFIV, definitely more than the $60 I paid for it.
I do have a bias though, I love Street Fighter a lot. The fans wanted an update and they gave us one. It makes sense for the fans and market wise.