There's been plenty of discussion here over the years of the merits and drawbacks of 'digital pianos' and other variations on the theme of keyboards synthesiser, but I thought it worth while posting a few comments on why these things are not, and will never be, a substitute for a piano.
Just so that people know where I'm coming from with this, apart from being a pianist I am a recording engineer and have extensive experience of designing, building and repairing electronic and electromechanical systems including all kinds of audio gear, so I do have _some_ idea what I'm on about!
Electronic piano-substitutes have lots of genuine, valuable uses, obviously, from (near-) silent practice to providing accompaniment for other musicians at piano-less venues. I occasionally get to play opera rehearsals on a keyboard, and while it's not as much fun as a piano it's arguably no less valid than a piano since what I'm playing wasn't written, originally, for either.
But there are two pretty major limitations. One is technical. I don't care how sophisticated your sampling is, _at best_ an electronic instrument can be no better than a recording of a piano, and any recording engineer will confirm that piano is one very difficult instrument to record and play back convincingly. Audiophiles spend many thousands of $ buying loudspeakers which still don't really reproduce piano entirely convincingly.
In addition to the recording and replay, there is the user interface - the key sensors. These are just as hard to get exactly right, and there's another problem in that there is a greater delay in an electronic instrument between the key going down and the sound coming out. It's only in the region of milliseconds, but it still gets in the way of the all-important ear/fingers/brain feedback loop that we all use when we play (well, when we play properly!). Speaking of time delay, many keyboards and all MIDI-controlled synths I've come across don't even play two notes simultaneously. They are only a millisecond or two apart but in a big chord this can be pretty obvious. (To test this, take a stiff straight-edge and us it to push down 15 or 20 white keys simultaneously. On many keyboards, you'll get a rapid scale rather than a simultaneous crunch of noise.)
And of course, as regards the recording and replay bit, where do you put the microphone? The sound coming from a piano varies _a lot_ depending on where one stands, note by note. A lot of the richness of tone of a fine piano in a good concert venue comes from the way the sound bounces around and mixes from various directions at the listener's seat. No synth I've seen has any like this richness and although it could be done it would cost probably about as much as a real concert piano!
Still, all those issues can in principle be addressed and I look forward to synth makers doing so as technology progresses.
But there's still a major blind alley intrinsic to the whole process of making a 'digital piano', and that is that it is an attempt to copy a pre-existing instrument. This is actually something new: when Cristofori invented the piano he wasn't trying to make it sound like a harpsichord! The electric guitar is a new instrument using the same interface and notation as the Spanish guitar, and the Hammond organ was never intended to mimic a church organ.
The nearest analogy I can think of is the Mona Lisa. You can photograph the Mona Lisa but the photo is still a reproduction, not the real thing, and no one will pay you millions for it or hang it in a major public art gallery. You could make a very detailed laser scan of the Mona Lisa and reproduce not only the colours but even the texture, and it would be a very impressive and interesting thing but it still wouldn't be the Mona Lisa. Similarly, a piano - like any instrument - is in its own way a work of art and however impressive a sonic copy is, it's at best a work of craft, and arguably hardly even that.
The big irony, it seems to me, is that keyboard synths can be wonderful things when they are producing sounds unimaginable before the age of electronics. There are some very fine keyboard players, especially in the worlds of jazz and rock, who play on keyboards stuff that would absolutely not work on piano.