> Black text has been fine since the first printing press
Keep in mind that paper is not an emissive surface, like an LCD panel is. Black-on-white is fine for e.g. e-ink displays or calibrated brightness (~15-30% on most displays, which is not the default).
Further, book paper - particularly for non-technical books, where diagrams are rare and reading is done in long, uninterrupted stretches - is often intentionally off-white, to lower the contrast and make it easier on the eyes.
Off-white paper is there for cost-saving reasons, used mostly for cheap romances and thrillers typically sold in airport bookstores. I don't think they even consider readers comfort when choosing it.
Smartphones tune down brightness in power saving mode (and so do e-ink books). Most websites are barely legible under those conditions, because web designers have been so heavily corrupted by backlight.
Keep in mind that paper is not an emissive surface, like an LCD panel is. Black-on-white is fine for e.g. e-ink displays or calibrated brightness (~15-30% on most displays, which is not the default).