So what is the logic behind Jhin?

This basic logic applies to all sorts of games.

When you cripple a character/item/ability in one area you give it a large advantage in another.

Jhin's entire kit should be based around being slow, ranged, and thinking about his fights but instead we got a finished champion that: Is horribly immobile both early and late ( and his movement speed buff will not help him unless the enemy also lacks mobility don't deny it), Has average AA range, Has decent ability range, abilities that only do well during the early and don't hit for anything during the late excluding the ult, An AD steroid (which is alright), but most importantly crippled auto attacked with majorly chunked crit damage.

For balanced purposes if you're going to cripple his mobility and attackspeed to give him a focus on damage then you actually need to follow through and give him a large buff in damage, instead the mess that we got is feeling less like a unique artistic adc and more like a really sub par ADC character who requires someone to baby sit him just to be able to do what other marksmen are capable of doing on their own with only alittle more damage to compensate.