I think it's a little naive to think that a digital keyboard will never be able to emulate a real piano to the degree that it is completely indistinguishable from the real thing as far as human ears are concerned, particularly when the argument for why it won't happen boils down to the sensors under the keys not reading enough information about the players performance and current sampling technology can't express the nuances in sound appropriately.

In other words, existing technology can't do it therefore it will never be done.

Frankly, I think advancements in the sensors reading the players performance and physical modeling of the actual sound generated by each hit, rather than simply recording and manipulating samples (or perhaps a combination of the two), may very well be able to recreate the capabilities of a real piano so accurately that you'll never be able to tell the difference.

Maybe it will take a long time (then again, maybe not given the general rate at which technological advancement seems to be increasing), but it's certainly impossible to say it will never happen with any degree of certainty. And for the laymen, or the person who plays simply to have fun playing some songs they love, it has pretty much happened already.