The shrinking between FF15 and REMAKE is negligible since 7R has no xbox version. In fact the first month sales of REMAKE is better than FF15 in USA.
As long as REBIRTH and FF16, it may be their products are not appealing. REBIRTH is part 2 of a trilogy which often sells least in a trilogy. FF16 is almost like an ACT game which is not expected by fans.
REBIRTH is more interesting because of its very good word of mouth, and in JAPAN it already shows better legs than FF16. Even in worst case REBIRTH won't sell less than Nier of SQUARE ENIX which sold 8 million.
This is...a lot lol.
First off you insinuate that Remake was at a disadvantage compared to XV cause of no Xbox version when... PS4 was at like ~48M when XV released compared to ~110M when Remake was released. I would hope Rebirth would have outsold XVs first month but it didn't.
Sure maybe in the US region it did, but I think people were talking worldwide. Which it didn't. As a matter of fact we got confirmation recently that in the US in terms of dollar (and unit?) sales, XV is thr best selling for the franchise in the US. So if what you're saying is true, XVs legs still maintained ahead of Remake in spite of that first month.
You're second point may be true with Rebirth and XVI but it's hard to guage appeal. If we went by purely the numbers that would make XII and IX and direct sequels some of the least appealing games in the franchise, and I am not all convinced on that point. I know for me personally XVI wasn't super appealing but I hesitate to apply that as a fact.
As for the sales prediction, I would personally love that outcome for Rebirth but I think that is hugely optimistic. History tends to show JRPGs tend to be very frontloaded, though there are a few that break that mold.
But yeah the first and last points really stuck out to me. I hope I am not coming across contrarian or argumentative but I did want to give a bit of pushback.