Except they've got an engine that works on both Cell and 360 hardware
Now, it's entirely possible that Catherine's basically held together with spit and blu-tak and would fall apart if the engine was pushed into being used for a different title, but I was under the impression that part of the point of developing Catherine was to give them precisely that option.
They're not using the Catherine engine anymore.
And, yeah, the Catherine project started out as a technical test for P5, which makes their sudden change of engine suspect. I think there's a good chance that the game either became next-gen or cross-gen with the engine switch. Not a sure thing, of course, but a good chance. A few reasons why before people ridicule me:
1. It's in Atlus' interest to create a bunch of SMT demon models that they can use for a whole generation.
2. Persona (and to a lesser extent, SMT in general) is a far bigger thing than it was when P4 released, especially outside of Japan. Unfortunately, the series has been relegated to systems that no one has or is paying any attention to outside of Japan (PSP, Vita, PS2 years after most people had upgraded and right before backwards-compatibility was phased out). If it came out on a popular system, people would pay more attention to it and be able to actually play it.
3. No backwards compatibly means that people would have to drag out their old consoles to play it, and the Persona audience probably overlaps a good deal with people who will upgrade in the first year or two of the PS4's life.
4. Being a next-gen RPG will probably get it a good deal of attention.
5. Judging from comments from the creators, this will probably be their most ambitious game ever.
6. I think Persona team is starting to think of themselves as world-class developers (and not undeservingly, in my opinion), as shown by their fancy new studio logo and stuff. Or maybe not. Just speculation.
7. P5 wouldn't be the only game to make the jump in 2011: both Versus and the Last Guardian (the latter heavily rumored) were switched to PS4 in that year.
8. The mods on GAF have been hinting at it for a while.
I could go on, but you get the idea. In short, P5 on PS4 is not a crazy proposition.