Jhin's power is greatly tied to lethality, and the reason for that is mathematically simple. For right now, let us assume that one statement, "Jhin's power is tied to lethality" is true. I will demonstrate why this is at the end, but for starters we need to make the argument.
Now, if we can assume that Jhin becomes stronger as lethality becomes stronger, and we can assume that Jhin becomes weaker as lethality becomes weaker, then we can also see an important point: if we simply buff Jhin's kit directly while lethality is weak, then Jhin may become playable, but if lethality is ever improved for another reason, Jhin becomes too strong.
For example, if Jhin is performing at 46% right now while building lethality, and we buff Jhin's kit so that he's performing at 50% while building lethality, that might be good for Jhin players. However, that would mean that if we buff lethality itself later because Talon or Zed is underperforming, then Jhin's win rate will increase above 50% while building lethality. That makes sense, right?
With that said, if Jhin is too weak and lethality items also look a bit weak, it makes more sense to simply buff the items knowing that that will buff Jhin because he primarily builds those items, rather than just buffing Jhin.
Now for the second part. Why is Jhin's kit so directly tied with lethality, when Jhin has more critical strike components than anyone else in the game. There are a number of reasons for this, but the most important reason is that Jhin's passive, Death in Four Acts, has an execute on it. This execute is likely intended to help Jhin farm effectively with only four autos per reload, and also serves as an interesting harass pattern.
If Jhin attacks someone while he has 300 AD, that AD does not directly improve his % missing health damage component. It does help him get their health lower for the fourth hit, but the fourth hit itself will deal only 300 + 25% of the target's missing health. 25% is an extreme amount, by the way. Many assassins only get 8% of a target's missing health, Jhin has triple that and it costs no mana and has no cooldown.
However, if Jhin attacks someone while he has lethality, that DOES increase the % missing health component, because it reduces the target's ability to resist the damage, making it closer to true damage. That's the reason why Jhin builds lethality and sees success with it. His % missing health damage in A Death in Four Acts is likely the largest amount of damage in his entire kit, and it has no AD ratio, cannot crit, and does not benefit from attack speed. So you build lethality. That's all there is to it.
nerf at the same time
while 
is one of the weakest champion in the game right now (46% win rate) we don't want him to jump back to 52% with his items to be buffed+the items used by his enemies to be nerfed at the same time
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