I worked on holographic data storage in LiNbO3 crystals in the early 2000 years. We had a lab prototype which would store over 1 GB in roughly a sugar cube size of crystal. The technology is certainly possible. However, the amount of optics required made it a huge and expensive system and there was not a lot of funding. Using modern diode lasers, the setup could be minimized a lot. However, the big limit is the wavelength of light. Green light has roughly 500nm of wavelength, and that determines the smalles features. This value back then was small but is huge compared to modern chip structures. We were losing the density battle even back then. There is one trait still not matched though: stability. Those crystals are about the toughest things imaginable. Unless you smash them with a hammer, they won't rot or decay. Storage times of hundreds if not thousands of years would be possible.