I'm not sure about this. Garnacho on the right has the strength (which I didn't know he had until he rattled the bar the other week from the RW) of being two footed. Ironically, contrary to what you're saying, the threat he carries is that he still does come inside (2/4 goals from the Right have been longer range left footed shots) but obviously defenders can't guarantee he will i.e. with Antony. He's also more dynamic than Antony which is the latter's real weakness, he's just too slow. Case in point, Rashford has played 2 full games on the right and even out of form has 1 goal.
If you look at Garnacho's goals form that side, he's scored with his right foot twice on the counter. Both the goal versus Villa and last one against WHUM were similar - on the break, ball played to him and a relatively simple finish each time. The longer range goals from his left foot, cutting in from the right are what we have missed. If you look at his heatmap he's not playing like an RM or RW either, so ETH is still asking him (or he's just doing it) to cut inside and create the overlap when we're attacking and the opponent has players behind the ball. The difference is Antony would get the ball in a lot of decent situations on the right but they would fizzle out, now Garnacho is actually causing more threat and getting a few goals.
He can still be a lot better, particularly regarding making simple passes to assist players - he does get the blinkers on when near goal - but the potential is there.