This sums up perfectly for me how difficult it is to be a referee, particularly at the very highest level.
I've watched that back from every angle available, in slow motion, and I still can't tell if it's a foul or not. There is contact, no two ways about that. But the question the referee has to answer is did the contact cause the player to go down. If yes, it's a penalty. If not, the player is cheating. When it's so difficult to tell even on replays, what chance does the poor guy have with one look from one angle at full speed? He would have my sympathies whichever way he went with it.
I don't accept that Sterling did what every footballer would do - the more we accept defences like that to justify cheating (if indeed that's what it was, only Sterling will ever know) the longer it will take to rid the game of such things.