The FGC wouldn't have died, no. But as much as people would hate to think about it, SF4 existing has helped more than it hurt. If not for that, I GUARANTEE, we wouldn't even have XIII most likely. KOFXII (if it would stay the same) would come out, bomb, and with the lack of a market, SNKP would go on with their actual plans to stop creating FGs.
Streams, heh, might as well give up on those. Sponsorships? Not to the volume we have now. MK probably continues to have no clue how to fix itself.
All that there would be left would be Blazblue MAYBE, Tekken and Soul Caliber. SNKP would be done, Capcom would just focus on other mainstream crap, NRS would fail to make a decent MK9 probably causing them to give up as well. Hell, there's no guarantee that ArcSys continues on.
You're right, SF4 not releasing wouldn't have killed the FGC, hell there'd be no noobs, you'd be enlightened. But ask yourself this...is it worth losing all of the above, on top of likely getting a better chance of getting older, possibly better or at least more beloved games rereleased?
I think you might be giving SFIV too much credit as well. Let's go back. 2006 sounds about right. Back then xbox 180 still had quite an online community. Problem was the online played beyond shitty. Capcom vs. SNK 2 had a decent fanbase, but if you thought any fighting game on the 360 has bad netcode then you ain't seen nothing yet. SF Anniv was even more popular with the SF 3rd Strike matches always jumping and even Hyper SF2 Anniv had enough people to get matches in from time to time. In SRK the online matches were probably even more talked about then, then they are now. True the famous people are talked about more, but you would have the top online players talking back and forth with each other and the other players always going back and forth nagging about this match or that match. When it really comes down to it SF 3rd Strike didn't really get ultra popular till the online community got a hold of it whether this or PC communities. Capcom was alive and thriving cause of online games.
The SNK community wasn't doing so bad compared to now. Back then KOF '03, and SVC lasted at least a year after release community wise before a significant slowdown happened. Sam Sho V, KOF MI, and Capcom Fighting Evo only lasted a few months, but that's par for the course for non popular fighting games now. Thease were considered the bad titles! On top of all of this Dead or Alive 2 was also being played a ton so the 3d community was well represented during all of this as well.
In came the 360. First came DOA 4 which sold like hot cakes. IT was a platinum title in no time flat. Street fighter II Turbo was the community breaker though. A title, which technically had already been released was selling like crazy and people were going nuts over the inproved netcode and quarter matches. This truly ushered in the fighting game boom IMO.
Let's just say Ono went to the bosses and they said we don't have a fighting game development team now and we don't want to subcontract SFIV to DIMPS. Where does that leave everything?
SFII HD Remix would still come out. It STILL would have sold like crazy, but what of Capcom now. IMO more then likely they would have make more HD remix titles. Alpha 3 and Marvel 2 would have been likely choices since a lot of it was not used cause making new 2.5D games are easier then respriteing & rebalancing their older titles. I think if we had more HD remix titles the community would still go nuts. There was a huge clamor for the HD titles after HD Remix was made.
As for other companies. Tekken 6, Blaz Blue, Soul Calibur IV, were already in the making. The game changer might have been KOF XII. My bet would be we would have never seen a rushed version of what we got if Playmore didn't panic. Also the simplified fighting game system would have never happened. What we would have got is something closer to XIII. Same characters (hackers already proved they are supposed to be in the game), game system close to the same as XIII without HD mode (did feel thrown in to me), same rebalanced system. IMO this would have gotten a lot more hype if it was XII, and wasn't lumped into a lot of new titles.
Main point I am trying to get across is the online getting powerful enough and having more options really fueled the online community. Not SFIV. Though it did get a lot of credit to get fresh blood in, the blood that woudl have stayed with fighting games would have probably have came in through another game eventually.