They are all good arguments Stras, and as you say, it has breen brought up before and most likely will again, especially if was 'offically' based on DEX. In my mind it could be based on each dependent on the situation. As a an advantageous move, its sort of like Defy Danger with a little spice on top.
I always try and imagine how I would handle it if a non-thief tried this fictionally? You already deal damage to an unsuspecting enemy without hack and slash, so the intent might be maximum damage or knocking them out or avoiding melee or somesuch. Effectively asking for one of the options under backstab, which would occur on a 10+ Defy Danger. On a 7-9 they would have a complication too. In which case it would most certainly fall under either DEX or STR, though INT or WIS is not out of the question, depending on the way the player presented their fictional actions...
So really, Backstab is an 'enhancement' or 'trained' iteration of a very specific (and contextual) untrained Defy Danger move. I don't know why its so scary to substitute DEX instead of STR for Backstab sometimes based on the narrative intent of the player. If it make sense, Go for it!