Why is it rubbish? Think about it. Sterling's main weakness before Guardiola was his decision-making in front of goal. I remember his final season at Liverpool very well. He was brilliant at Liverpool I agree but he was still hesitant in front of goal. You even agreed with that yourself.
Compare both of their stats:
Sterling's final season at Liverpool PL stats at 20;
35 games, 7 goals, 8 assists.
Rashford's PL stats at same age (17/18);
35 games, 7 goals, 5 assists.
Sterling's 1st season at City PL stats at 21;
31 games, 6 goals, 2 assists.
Rashford PL stats last season at 21;
33 games, 10 goals, 7 assists.
Sterling's 2nd season at City PL stats at 22;
33 games, 7 goals, 10 assists.
Sterling's PL stats with City at 23;
33 games, 18 goals, 15 assists.
As you can see, their records at the same age are very similar.
To say Sterling was always destined for greatness wasn't true & is hindsight. Nothing is ever set in stone. I never thought he was capable of the records he's posting now, that's for sure. He's benefited from excellent coaching at City from Guardiola and has matured with age.
Rashford has similar weaknesses. He is excellent at getting into a dangerous position, but once there, his decision making is letting him down. That's normal though. The biggest thing is that he keeps on getting into those positions in the first place.
That's how it is with young players, they consistently frustrate until bang, they explode, it suddenly clicks and away they go.