Read more10% slow on W is eating away whole tier of boots. Even at lowest MS it's minus 33 units moved each second, and her W lasts 2 seconds. 10 mana off means like 2 more rockets or 1 more cast of W during the laning phase which translates into like 100 more damage (post-mitigation).
AS is also more important stat for ADCs, cos it translates into mobility (you stand still for less performing an AA).
And that's not mentioning Jinx being able to outsmite junglers again (when the reason to up t3 Smite to 1200 was exactly that: it was too easy to outsmite junglers and you wanted to make junglers more important when fighting for objectives, that was one of the goals of jungle changes this preseason). Also old cap was 800, so still less than Smite, now it's 1200, just as strong as Smite, so relatively it's stronger.
Also-also: 13.7 Kalista vs 13.8 Kalista (buffed by +0.25 AD/lvl):
...
Kalista was buffed in 13.7, not 13.8. She gained about 0.6% win rate in 13.7 with a 10% (not percentage point) pick rate increase.
So... Looks like I'm right on the money.
Let me help you with some English lessons: https://www.dictionary.com/browse/placebo