For how long is Sion going to have a 54% winrate while maintaining a 20% pickrate?
Sion is currently THE best toplaner and arguably the strongest any toplaner has been in soloQ for the past year (relative to their respective meta). He boasts excellent stats (55% winrate, 20% pickrate and 40% banrate in diamond, even better below) and performs very well against everything.
The worst thing about him is that unlike previously dominating toplaners like GP and Camille in this season he simply lacks a weakness. Against GP and Camille people could at the very least make the excuse that they had certain bad matchups, however that is not the case with Sion as even his worst matchup is decent at worst.
In terms of gameplay he shows to be strong at any stage of the game: in the early game he manages to enable his jungler by having the push while still being relatively safe. Any attempt to engage or trade is futile due to the minion aggro which makes ganks very easy to predict as trading is generally not an option against Sion so if anyone walks up chances are that the jungler is approaching. That makes it so that he generally is extremely tanky even at the start of the midgame while also being able to just shove and roam with his ultimate - all without losing farm and forcing his opponent to either surrender the map pressure or the wave. And then finally he scales into being an incredibly tanky, CC-heavy tank that can easily peel or engage while controlling the sidelanes with ease.
There is just no reason to not want to have a Sion on your team no matter the comp - at worst you will have the slightly better toplaner at best you will have a toplaner that dominates from start to end. Only one question remains: where are the nerfs?