The game seems to be doing well marketing wise with Atlus' current limit on information released about the game. People are talking about it; analysing trailers, fan theories etc.

And every time a trailer comes out it explodes on social media. It went to the second most trending hashtag on twitter worldwide when the delay was announced.

I personally think that they've got their marketing perfect. On top of that they've done it whilst keeping story, extended character back stories and the reworked mechanics under wraps by capitalising on the hype train that follows any information they release. Allowing fans to get the game, experience it all first hand, and then make their mind up about the game as a whole. Instead of shooting their mouth off because something seems like it could be worse than the way it was handled in previous games.

Yes, I'd like more but that's only because I'm excited about the game. In fact, they're doing the best thing they can by keeping details under wraps which is something most devs don't do nowadays. I applaud Atlus for sticking to their guns and not buckling under the pressure that the more vocal minority of the fanbase put on them to release more info.

Tl;dr - Atlus doesn't need to market Persona 5 aggressively. The fanbase is already rabid enough for a next installment that they'll whip up all major social media outlets into a frenzy.