Since people keep bringing up install base and I'm saying in this moment it doesn't matter. I'll prove my case using FF13 trilogy.
PS3 install base when FF13 launched: 4.3 million. Sold 220k during FF13 launch week and 75k the previous week.
FF13 launched with 1.5 million on that 4.3 million install base.
FF13-2 launched with 520k sales with an install base of 7.5 million.
Lightning Returns FF13 launched with 277k sales with an install base of 9.5 million.
There was way less interest in the newer entries. The same can be said with FF7 Rebirth. FF7 Remake was very decisive, so obviously a lot of people would have no interest in it's sequel. It doesn't help that since FF13 Square Enix have "tainted" the FF franchise for Japanese fans. Final Fantasy used to be massive in Japan, now it isn't. This isn't an install base issue, it's a franchise issue. FF7 Remake sold 7 million in 3+ years. I do not see FF7 Rebirth coming close to that.