Why wouldn't you? Interest in general does seem much lower, especially in Japan. I think success stories like Persona 5 distort expectations of the games which represent the ceiling. I've talked about this before in many previous MC threads, but the trend we're seeing with Japanese console game sales post-FFXIII was that the floor started to rise, while the ceiling continued to fall. Which is to say, as the PS3 recovered from the disastrous early days, games which sell at the low end or the mid range saw their sales increasing again. So we saw that it became possible for popular games to sell 500k, then 600k, and "big hits" started selling up to 800-900k even. But yet all this time, nothing on the PS3 ever managed to break the 1 million barrier again with a single release. FFXIII remains the only game in Japan to have ever sold over 1 million copies on the PS3 with a single release. That means the ceiling dropped.
The success the PS3 had in recovering the console market didn't translate naturally to the PS4 either. It took a long time for the PS4 to become relevant in Japan, and it still struggles to this day. Persona 5 is a great success for Atlus, but let's not forget, it's not 336k in PS4 sales. 21% of the first week sales went to the PS3 release - a dead platform. Stuff like this just don't indicate a very healthy market that is full steam ahead in embracing the current consoles as must haves. These are factors which will be obstacles to the success of a game like FFXV.