He's very quick no doubt but do you think he shows enough aggression with his quickness in terms of making runs in behind to threaten the FB and set the whole back 4 deeper by a few yards?
I think Kulu is much better than Johnson in terms of beating his man but he also, imo, offers technical security when he receives and plays the ball.
Johnson would be an excellent option off the bench if he showed the desire to make runs in behind that threaten the FB, if he tried to take on his man for pace more often because that's the only weapon he has, if he was able to receive the ball under pressure and keep it, if he was able to make combinations with teammates both in terms of passing and his movement but in my opinion it's too many if's for him to be considered a relevant piece in a team that's competing for trophies.
As long he's a starter or plays lots of minutes what this team can achieve is limited imo.