Well, not much longer. SF4's last update was in 2014 and it came out in 2009. SF5 came out in 2016 and it's last update is (hopefully) this year.
I still feel it has more to do with console gaming growing as a whole and repurchasing. Plus the fighting game culture/esports in general has grown massively.
Street Fighter IV was on Xbox, PS3, and PC and V still out sold it in less time
Street Fighter has always been a game that lasted years
They just made bunch of versions of Street Fighter II
Street fighter II
Street Fighter II Championship Edition
Street fighter II Hyper fighting
Super Street Fighter II
and Super Street fighter II Turbo
they had to make nerfs and buffs in whole now games to balance it out back then, now you can just download the update and add characters in DLC
Street Fighter III even though purist say its the best in the series, was a low point for the series, arcades were dying, and street fighter was losing popularity after the popular alpha series and vs series
Street Fighter IV restored the tournament scene and brought fighting games back up into Esports and made EVO a bigger thing than what it was
so V being more popular than IV is an accomplisment