Everyone is entitled to a period of poor form, but for me, that is something that lasts a few weeks, or at worse a couple of months. Sterling has been out of form for far too long.
KDB is widely regarded as the best midfielder in the world (and rightly so), he is having a below-par season by his own very high standards, but is still 2nd in the assists chart. Apart from a couple of recent games, he hasn't absolutely stunk the place out despite being below his normal high level. We are used to him being head and shoulders above everyone on the pitch, but this season he has been just another one of our very good players.
Sterling on the other hand, isn't just significantly behind where he has been in goals wise, he is IMO more often not our worst player. He has found zero rhythm to his game all season, and doesn't deserve his place in the side. Some think this is an outlandish claim, but when was the last time he put in a performance like Foden or Mahrez did last night? People are obsessed with saying that he would be difficult to replace, but forget spending £100m in the transfer market, there are players already at the club who improve significantly on the Sterling we have seen this season.
As for him getting back to his 17/18, 18/19 levels, I have no idea. We would all love that to happen.