First of all, please forgive me for my English, I am not a native speaker.
During the last 2 years I have mainly played ADC and since the beginning of Season 7 I noticed that the difficulty of playing this role has been highly increasing. Not only me but even some pro ADC players agree with this statement.
In my experience, ADC is the most diffucult role for what concernes mechanics, due to the clicks-per-second, kiting and so on. Because the role is
mostly mechanical based, even the smallest mistake is super punishing. This concept is good for me, because it underlines the high-damage/high-risk idea behind marksman champions.
I'll try to explain what I understand of League in order to make my point clear.
General ADC: KIT /BUILD
AA Based /Full damage
High scaling damage /Maximum 2 defensive items (Zhonya, GA)
No mobility /0 health items
So I think that the idea behind this kind of champion is: big damage guy, squishy, needs HELP FROM TEAM MATES in teamfights due to its kit and build, while others roles don't. This is the point: strongly depends on others.
I am aware of the fact that one entire role exists for that, the support. But nowadays is more WORTH to peel for the top-laner/jungler/mage insted of the ADC, because they are more useful in early stages.
League of Legends is a TEAM game splitted between normal game (SoloQ) and pro-players game (LCS, Globals,...). There is an epic difference between these 2, especially for ADCs.
Because in pro-players' world ADC is well protected and everything runs smoothly but the 99% of League's players are normal, and in SoloQ GENERALLY (only the AVERAGE matters) people don't care about peeling for others, they play their own game. It doesn't matter if it is not like Riot designed it, if it is not how it is supposed to work. This is the reality.
Sometimes the correct positiong for an ADC in a 5v5 fight is not being in the fight. In my opinion when that happens it means that something is wrong.
What I have just explained seems reveal that is not a problem of champions but is a problem of how the whole team plays with them, but on even conditions (before 6 completed items) a marksman can't win a fight against an irelia, for example. And this is just math.
ADCs, in a team, should fill the position of doing the most damage to the other team, paying the price of being vulnerable when on its own. However, in this meta bruisers like Irelia, Camille, Fiora, Yasuo and many others has damage (even in late game, 6 items) comparable or even more than ADCs, but without being as vulnerable as them. So which is their role in a team? Snowball carries?
Bruisers' kit now has CCs, high damage and mobility, and they build health like Sterak's Rage without losing damage as well. That is why the comparison cannot hold. Bruisers surclass ADCs in almost every point, and with almost none disadvantage.
For instance, Jax 12/0/0 can win a 1v3 by himself while Caitlyn 12/0/0 explodes in a second with any CC.
I have no doubts that marksmen can be easily substitute in botlane with every other kind of champion still providing better performances, such as Heimerdinger, Karthus, Yasuo... because in this meta any marksman (but maybe Draven) needs at least 3 items to reach a power spike, while Irelia just needs Trinity Force.
SoloQ is fast in this Season, so ADCs often don't have enough time to scale and be performing like bruisers, which scale faster (less items needed).
After this long speech, I hope that you agree to the fact that the issue regards time, which means gold, which means items.
Considered the kit and the build as I explained before, Irelia/Camille/Jax with Trinity Force destroys whatever ADC with Stormrazor+boots. So I hope it is clear that this problem only regards early and mid-game.
My personal solution is: don't buff ADCs' dmg, kit nor scaling. I think that they are fine on their own. But nerf bruisers' flat dmg. I personally suggest to transform their kit in something more percentage of enemy's health. In this way ADC shouldn't be deleted in a second but bruisers can still be effective against tanks.
The idea behind this is: as ADCs need protection from their team, in the same way bruisers need coordination with their team to kill ADCs.
In conclusion, I just want to say that the right question should be: What is the champion's role in a team?
Because marksmen are not carries in this moment.