If I had to guess I'd say they originally began work on a PS3 P5 release but scraped it in favour of next-gen tech that Sony must have given them some time in the past 2/3 years (I've heard plenty of rumours that companies have had the tech for quite some time).
This would put P5 in line with all the other next-gen games' dev cycles but would hopefully also explain why we haven't heard anything for so long.
No. I'd be surprised if Persona 5 isn't on current generation hardware. P3 and P4 were both released on the PS2 well after the PS3 was released. Atlus hadn't even made a PS3 or Xbox 360 game until Catherine.