The actual latency budget would be less than a single frame to be completely non-noticable, so you are in fact limited to less than 1 GB to move per each keystr

The actual latency budget would be less than a single frame to be completely non-noticable, so you are in fact limited to less than 1 GB to move per each keystroke. And each character may hold additional metadata like syntax highlight states, so 1 GB of movable memory doesn't translate to 1 GB of text either. You are still correct in that a line-based array is enough for most cases today, but I don't think it's generally true.