[kps]

  > notwithstanding that the ASR33's concept of Rubout didn't
  > really map at all onto the new hardware.

If you are working with paper tape, Rubout (DEL in ASCII parlance), like every other code, advances the tape when punched. So, if the tape is a stream of characters, DEL erases the one under the cursor and leaves the cursor on the character formerly to the right. That is, Rubout/DEL is defined as a ‘forward delete’ operation, and that's something that remains useful.

That leaves Backspace as the natural choice for entering ‘backward delete’ on a keyboard, at least after 1979 when you have the ANSI X3.64 escape sequences for explicitly nondestructive cursor keys. I think there's a reasonable argument for Backspace being nondestructive for overstrike effects (accents, underlining, APL, etc.), especially when received by a terminal, but I know none for changing the meaning of DEL.