Just so you know Double Helix isn't developing the game anymore. Also we know the quality of KI, just look at metacritic or read peoples impressions. Now we have just to wait and see whether SV5 will be good.
I thought SFIV was amazing so to be completely honest I don't need to wait. What I'm trying to say (my main point) is that for people who like fighting games, Killer Instinct might just be enough. It's not for me but I am certain KI could fulfill enough of a fighting game need for most people.
It's not the huge genre it once was, and SFIV's popularity was a shocker at the time, though I was one of those who couldn't wait to play it. I also personally liked Street Fighter III but didn't play it much.
I guess on both accounts my point is that SFV may see the same lack of interest as SFIII due to the popularity of their respective predecessors. And also, there are fighting fixes that can be somewhat met on other consoles with other exclusives. It all depends on how big the SF craving will be when SFV comes out. Because we all know it will be great.