I didn't say I thought they rewrote the engine from scratch, just that I think it's more than a couple bug fixes. I think they did some digging around and made some significant changes. That doesn't mean they trashed everything and started over. They did re-do an awful lot of the graphics and sound assets.
They really didn't change that many graphics from OG SFII to SSFIIX. There were a few basics added (Chun Li got a new standing high kick at some point, Super, I think), There were some new moves like Chun Li's Kikouken (which was originally made from existing frames in SFII' Turbo, but got its own animation in Super). Everyone got a super in SSFIIX. The endings changed. The re-drew Ken/Ryu's hadouken at some point. Ken has a flaming fierce shouryuken.
When it comes to things like standing, jumping, punching kicking, win pose...the stuff that makes up the vast majority of all the graphics, that shit hardly changed at all. Less popular characters like Guile and Zangief are almost indistinguishable between World Warrior and Grand Master Challenge.
The SFC version didn't update the original character graphics much, if at all, but the arcade did for many of them. So I you are right, in one sense. If they did just another port using the SFC graphical assets it wouldn't be as much change as if they did an original arcade port. But if you're going to create an ACD game, why settle for a SFC port? Why not do it one better and actually show off the hardware? If it's just the same game as the SFC version than why would anyone bother getting the ACD version, unless they just didn't happen to have an SFC?
Ok, seriously, do you actually know Street Fighter very well? I'm obviously not talking about an ACD port of the SFC version because there IS no SFC version of SSFIIX to begin with.
The latest SFC version is just regular Super, not Super Turbo, and it is a more or less complete version of that game. Everything that was changed going from Turbo to Super in the arcade (new sounds, new fireball animation for Chun Li, flaming shouryuken, dorky new "Tiger Knee!" sample for Sagat, new endings) was also changed with the SFC version. I can't think of anything that was left out. It obviously doesn't have any of the Super Turbo changes because it isn't Super Turbo. The only home versions of Super Turbo (then, anyway) were 3DO and Marty.
I'm starting to think this is a matter of perspective, really.
Its like this. NEC had SFII', and it was mighty good. In order to make SSFIIX all they had to do was edit the new elements in.
It wouldn't make any sense to build it from the ground up since the project was already %80 done.
It *really* wouldn't make any f*cking sense at all to use any aspect of the SFC version since it isn't Super Turbo. You'd have to port the thing to PCE, re-format it so it would load from a CD, then add all the Super Turbo assets (the super moves, the new sounds, the endings) in because they don't exist in Super.
It makes a lot more sense to just port develop a CD version of the game they ready had and did such a good job with, and then add everything that changed from SFII' to SSFIIX in one go.