If RS and US were in common use, there would be a need to have a visible representation for them in the terminal, and a way to enter RS on the keyboard. Pretty soon, strings that contain RS would become much more common in the wild.
Then, one day somebody would need to store one of those strings in a table, and there would be no way to do so without escaping.
I do think that having RS display in the terminal (like a newline followed by some graphic?) and using it would be an improvement over TSV's use of newline for this purpose, but considering that it's not a perfect solution, I can understand why people are not overly motivated to make this happen. The time for this may have been 40+ years ago when a standard for how to display or type it would be feasible to agree upon.