Let's not forget the actual changes and not the ideals with the ADC update
%bonus pen means SQUISHIES are harder to kill. Let's also not forget that AD is getting lower across the board to the point that you can expect about 20-30% less AD at lvl 18 and more before then. Don't believe me? Check all the high tier AD items. They've ALL been lowered from 80 AD to 60-65 AD.
Crit don't mean shit if your underlying attacks aren't at least marginally useful. And before you say that the crit increases will cancel out the AD increases, AS is also being lowered across the board except in a few items. The ubiquitous Zeal that has been a cheap stable for cost efficient midgame dps after an AD item since SEASON 2 has had its AS halved. Lots of AS items have gone from 30->20, 55->30, 50->40.
The AS is being lowered in such a way that it itself either completely or mostly mitigates the crit buff on top of the reduction in AD. Even the base AD of ADC are going down in the mid and late game by around 10-15%. Every ADC is getting +5 AD at lvl 1. Great! Except the average reduction in AD/lvl is about 1.5. It's insane. Crit might be awesome, but the base damage of these autos will be shit compared to before.
But ADC now have awesome effects and help vs tanks and bruisers. There's +15% physical damage vs targets with 500 more max health than you. Then several armor items are getting some minor nerfs for mid game, like warden's mail with 50->40. Many armor items are 50-300 gold more expensive depending on their tier.
Basically, tanks are becoming squishier vs physical damage while also receiving several MR buffs (yay!). Simultaneously, the base dps of marksmen is getting nerfed significantly. They then recover a lot of that lost dps vs beefy targets through the bonus damage and conversion to bonus armor pen. In fact, both of those effects are on the same item!
Marksmen are getting more unique effects, but it's at the cost of raw damage. Shiv has its own niche, but it's not like the burst from before. Runaan's might be worth buying on more than 3 champions now. Maw is potentially a really good defensive item for ADC because of the shield change.
What's in the back of my mind is that even though MR items are getting some decent buffs, they're only for tanks or bruisers. Squishies don't usually build them. AP damage is virtually untouched except for some mages in the early game. That means that mages' damage has been largely the same while the underlying base dps of ADC has been reduced.
Make no mistake. I don't think ADC are getting nerfed secretly and that you're all stupidly supporting Rito. I think Riot's going ham with this update and putting their all into it, and I think it'll work out well with ADC being more varied than they are now. This is by far the preseason with the biggest changes in 5 years. SOMETHING good is going to come out of it.
I'm just waiting for day one of the preseason and all the AP mid mains are going to dominate all the ADC that think the itemization changes made them into gods.