Regarding Nords and pianos...
I sold a Stage 3 88 to fund the Prophet X, but no, it cannot replace an Electro or Stage, they're different instruments.
Nords have tons of polyphony available, whereas you can easily exhaust the 8 stereo voices in the X. Nords are also optimized for live performance, the excess polyphony is used to keep the last program going, even after you change programs. On the Prophet X, all notes stop when you change patches. Nords also have seamless transitions now, with three adjustable widths, which is wonderful live, or in a piano bar. So if you're in a jam band, you're still going to prefer your Nord. However if you're in a studio, or you're a band that pauses between songs, the stereo filters and effects will likely win you over. Nords sound good, especially for the size of their samples, but they don't sound like a Prophet.
As to whether you can play piano on it, it depends. If you're playing one handed riffs without suspend, sure, it's capable. However if you're trying to play Beethoven, forget it. You'll run out of voices, especially if you're using sustain. I learned how to play the first movement of Moonlight Sonata on a Nord Piano 3 and Stage 3 88 and it's not practical on the X, nor did I expect it to be. In addition to the Prophet X, I also ordered a Kawai MP11SE, that was my solution. So if it's mainly piano or organ that you're after, you're still better off with an Electro or Stage. The Stage is also a better MIDI controller, since it can control two separate external zones, and there's the (now slightly broken) DualKB mode.
One other observation. My Nords always sounded better in headphones than they did on my studio monitors. Over in the Nord forum, you'll find a lot of discussions about appropriate amps to pair them with. I've always suspected that my Mackie HR824s were not up to the task, judging by what others use with their Nords, whereas the Prophet X sounds lovely. I'm not sure if it's the analog stereo effects and filters or what, but it's there. The Prophet fills the space in a way the Nords didn't.