ADCs are completely irrelevant until 4 maybe even 5 items. Something has to be done
ADCs are in a weird spot. I won't say bad spot per say because they still scale extremely well if you can somehow last to 4 or 5 items.
the problem is as an adc player you basically are at the mercy of your teams moreso than any other role right now which feels bad for a carry role. I want to preface this by making it clear that I think the role doesn't need to be what it was with lethality running rampant in the mid game, but it certainly doesn't need to be what it is now.
Here are my thoughts on why ADCs are taking way too long to scale currently.
1.) Anti-ADC Itemization is too strong/cheap
currently in the mid game you just need a health component and ninja tabi and you are too tanky for an adc to do much of anything to you since they will not have the damage to get through the tabi. Tank items are naturally cheaper than damage dealing items and far more efficient. this has been the norm for ages. But now that tanks are getting shields galore either through over buffs (cough poppy cough) or through colossus (hello anyone with any form of hard CC) the items give too much. The items imo were previously balanced where tanks reign supreme mid game, but were not opressive except for the height of sunfire/iceborn era. These items combined with colossus now are creating tanks that are simply TOO tanky for adcs to do anything in the mid game. I'm not advocating that ADCs should be able to melt tanks in the mid game, but if we're being dove under our tower kiting for a slew of autos and tower shots it should at least feel like we made a dent in the tanks health as opposed to tickling them.
2.) ADC Itemization needs tuning
right now, after lethality has been changed, adcs really have only one viable build path: Crit > armor pen/lifesteal. the problem is the items are very expensive and require all of them together to really do much of anything. You need IE, Zeal item, Life steal, and armor pen to really do much of anything sustained in a fight. and nevermind that you at least want a component defensive item in hexdrinker or a QSS. Currently Itemization just straight up sucks. its awful and buying any one item individually will never be enough for mid game. bork by itself sucks, bt by itself sucks. Ie by itself sucks. you need at least 3 items to do damage and 4 to really do any sustained damage because without life steal you aren't going to able to stay in a fight for very long.
3.) Fervor is geared way too much towards late game.
Fervor giving raw ad now instead of on hit is an interesting decision but it means that until you get crit and attack speed its not going to do much if anything for you. this leaves you with two options. play for the late game or go warlords to save yourself from having to buy a lifesteal item early. (lets you spike 1 item earlier at 3 items instead of 4). Warlord's doesnt even feel OP. fervor just feels like it scales well, but takes too long to be relevant. it needs more early game power in some way. I'm not sure how you do this, but something has to be done.
Now I want to finish by saying I don't think ADCs should be early and mid game monsters, but the power curve of useless until 3/4 items and then god mode is just not fun currently. its by far the most frustrating role to play right now since literally every other role is more relevant than you until 30 minutes or so.

(1100g) +
(1300g) +
(2600g) = 5000g