I can see that and my shot in the dark guess is PS4 will probably end up around N64 numbers (~5m) give or take. But taking a massive turnaround like this for granted still undersells just how disasterious the platform's debut has been. It's the worst debut of any major Japanese platform we have data for.
I'm also a bit hesitant to look at software trends from a half decade ago and assume it's a given those franchise will still maintain the same hardware moving capability. We fell into this exact trap with Wii U, which despite having essentially the same sort of lineup as Wii sold dramatically different. It's very possible that rather than lifting PS4 to mainstream heights, PS4 will simply drag these franchises (FF, KH, MGS, RE, WE, Musou, Yakuza, etc) down proportionately with it. Just like Wii U did for it's lineup (NSMB, Wii Party/Fit, Mario Kart, Smash, DKCR, Taiko, DQX, etc).