By balance, he wasn't a problematic champ like fiora has been. But like master yi, he was intentionally gated in power due to how overbearing his kit was. Full AP sion had a nuke on a 5 second CD. Full crit could heal for 2K hp per auto. Similar to taric and WW with their point&click CC, he was an unhealthy stat stick.
See the flaws with the skins point. However think about the lack of skins, had new sion been released as a "new champ". Who would get mecha zero?
By overhauls, I mean the drastic changes (the jungle, reworks, items, things like vision/stat tweaks). Think about how much each pre season has trampled over what we could see clearly in the mud.
That being said, they didn't even TRY to work with him.
They likely did, relentlessly. They gave you a toned down version of his Q in his new E. His relentless charge was replaced with his ult and the W, while relatively unexciting stuck around.
The likely conclusion was "there is no way to completely preserve this champ without him feeling like total shit". I mean look at poppy. How the hell do you preserve that old ultimate?