I think sterling's probably hit his ceiling. There's only so much Pep can do, he's worked wonders with him so far but sterling's speed of thought, decision making and control are all piss poor, that's pretty much undeniable now and they're what's stopping him improving further and are things that are likely never to improve now, at least not enough.
Our moves break down with him so often now and not because he's trying something risky where you accept it might break down like if say kdb tried a through ball, they're breaking down with him failing to do the basics.
I think we're at the stage now where replacing sterling is high up on the list of ways to constantly improve our side. Imagine you're txiki evaluating the squad, you look at replacing mendy and finding a successor for aguero/fernandinho as the priorities in this squad and then after that if you look at our squad and say where can we improve the most, there's more scope for that in replacing sterling than anywhere else in the side.