From today's persepective, maybe. However, it is entirely consistent with the terminal control sequences of that era which can't use CL for transmitting data.

From today's persepective, maybe. However, it is entirely consistent with the terminal control sequences of that era which can't use CL for transmitting data. It's limited to control, or else you wouldn't be able to cancel operations at arbitrary points. Sixel data is using GL, CR and GR [the high six bits], and from the point of view of fitting in with the design constraints, it's actually quite elegant.