Language tends toward streamlining. Irregular verbs smooth out, redundant punctuation disappears, and open phrases inevitably fuse into closed compounds. Over hundreds of years, today lost its hyphen, electronic mail became email, and every one became everyone.
Yet orthography possesses its own defensive immune system. When visual compression disrupts the fundamental link between how a word looks and how it sounds, readers push back. The two-word construction of no one preserves phonetic clarity, protects the integrity of the long 'o', and gives the eye room to breathe.
Digital informal typing will continue to push noone onto screens through raw ergonomic laziness. Until predictive systems figure out a way to resolve the visual collision with noon, traditional grammar stands firm. Keep the space, ditch the hyphen, and leave the closed compound to informal text threads.