plenty of casuals continue to play games like smash brothers, starcraft, and league of legends for years
LoL yes because it has massive post release content.
Starcraft has long been started to get phased out.
Smash Bros is an anomaly (well Melee is, Smash 4 had a lot of post release content). And it's arguable that people who have been playing Smash for over a decade aren't actually casual, it's about how many new people are coming in (only LoL gets newer players in regularly).
I mean USF4 held in there with post release content too but once that is stopped, the casual players will move on too.
Point is that his will not apply to SFV for years because barring some massive failure it will get supported. Has less to do with single player content (like all the MOBAs, CS GO etc).