We don't have anyone who makes the runs Son does on a regular basis. Sterling will do it occasionally (but playing the same way with him as we do with Son would be a bad idea anyway, he isn't clinical) yet still not enough. Most of our pacey wide players want it to feet so they can dribble.
Rashford would've fit the bill as a Son type player, not as good obviously but a couple of years ago he was looking like a discount Mbappe. Fast, powerful runner, quick feet & good finisher. Now his game has gone to shit.
Son isn't clinical either. He's hot and cold. Let's not pretend that for all the goals he scores, he doesn't also miss chances. He has games where he is absolutely rotten. His numbers would be insane if he scored the majority of the chances he does get. But as a wide player he can be forgiven, and so can Sterling.