4 years ago he was 19. He shouldn't have been a first teamer but it wasn't his fault. His pace was useful then and it is useful now. He is just a much better player at this moment and has shown progression like any other player. We see ability in Chamberlain but he struggles with consistency too, he just doesn't get the abuse Theo got from some because he obviously is technically more superior than Theo was at 19.
Theo was scoring 2 important goals at Birmingham 5 seasons back at the age of 18, in a match his goals should have won us. He has always shown end product. Wenger will argue that if we see a more rounded player now, it is because he has been given time to play from the age of 17. That policy is a different debate but Theo doesn't deserve abuse for it.
I think he has worked quite hard to get where he is today. His all round game has come leaps and bounds but really whether he was useful with his attributes should not be in question.