People are looking at the previous sales of Street Fighter 4 and all of its versions, but they're forgetting everything in between Street Fighter 4 and Street Fighter 2 that didn't sell well. Capcom milked the series so hard in the 90s that when newer versions of the game came out, people just didn't care and I feel the same thing is happening again with Street Fighter 5.
I think this is how it's going to turn out. Casual gamers or casual fighting game fans don't play fighters to perfect their technique and they'll probably play it around a week before their friends and them finally give up on it and move to something else. Those interested might play online a bit, rage after several losses as they're unskilled scrubs and quit. If this has happened to them a few times in the past few years I'm sure it's stuck within their memory and they'll ignore most fighting games. Being reminded with constant releases of Street Fighter 4 and how quick they got bored of it isn't doing any favors for Street Fighter 5.
Street Fighter 5 is going to sell worse than just the PS3 Street Fighter 4 combined sales.
Good point. It's not for nothing that SF4 took so long to be released. I don't remember SFIII being super popular on consoles for people who weren't really into the series.