People are not looking at the FFXV sales rationally.
First of all, let's estimate the release window of FFXV in Fall 2016 at the earliest. This mean the PS4, which has currently sold 1.5 million in Japan, should be at a LTD of somewhere between 2.5 to 3.5 million.
Now the question is, how much can it sell to the fanbase of 2.5 to 3.5 million (if it is out by Fall 2016). I am confident that even at these numbers, an opening of 1 million+ is possible for the debut week. However, it is definitely hard to see it reaching FF XIII figure.
By contrast, both the PS1 and PS2 were extremely popular sales wise and hence the better debut for the past FF games is no surprise. So before people start blaming the series, look at the hardware market first too. It has nothing to do with series sales, but everything to do with the declining sales of dedicated hardware in Japan.
Speaking of the sales worldwide, I can say this with confidence that it will easily sell FF XIII, unless SE somehow fucks up its marketing and the end game is critically panned. A critical acclaim will also help a lot with sales, and we can see the same with The Witcher 3, which sold loads more just because of critcial acclaim.