I think you overstate the importance of day 1 sales.
SF3, the game that "killed the SF franchise" (nonsense, btw) had 2 chances to add a bunch of fan favorites in order to attract casuals.
They added Chun and Akuma...and a naked man, a cyborg dude, a tom boy, a bishonen, a puddy man, and an Andore. No Sagat. No Sakura. No Guile. No Blanka.
Capcom has a history of not giving a sh*t when it comes to character additions, regardless of sales. They openly admitted to adding Adon in SF4 even though they knew no one cared about him. Capcam adds who they want, when they want. If they want to add Remy to SF5, we're getting Remy.
And anyways, reading interviews from the developers of SF5, it doesn't sound as if they're panicking anywhere near as much as GAF thinks they are. They've stated many times that this is a marathon, not a sprint.