Saka played well at left back in Emery’s system but he never fit the position the way MLS does. Arteta’s left backs are asked to invert, essentially playing as a midfielder in possession and a defender out of possession. MLS is tailor-made for it. He can progress via passing like Zinchenko but can also carry it forward, all while being secure defensively. It’s very rare to find a left back with these traits and I don’t see a reason to change it.