When you translate to MIDI, you are going to lose a lot of the subtle pitch and tonality of the piano. One string vibrating produces overtones and a chord will vibrate in a very deep way on a grand piano that I feel you cannot replicate in a MIDI, and therefore that detail would be subsequently lost.

For instance, how long you are touching the string - while you're touching the string, there is a sound - but after you let off, you get the "reverb" - and there is different reverb for how you hit the key, if you bounce, or if you stay for a split second longer for staccato, I don't feel like these subtleties translate to MIDI.

It is certainly easier than violin, that I will grant.

edit: IMO the best way to do what you are describing is get a really good pianist to sit down and do the work. I don't think that (current?) machine learning can really "understand" the nuance of phrasing esp that would be coming from older recordings.