The problem is unflexible build paths for ADCs, though this applied to many other champs as well.
With a few exceptions, they all go into crit or on-hit. On-hit provides lifesteal and damage. Crit provides just damage.
Currently crit is basically the only way ADCs scale into late game. Certainly on-hit is strong as well, but usually on-hit champs build crit afterward anyway so they can kill squishies.
This entire build tree (crit + on-hit) offers nothing in the way of health, CDR, mana, armor, magic resistance, etc. If you build anything else except crit or on-hit you are delaying your multiplicative powerspite. On-hit users are more flexible in this regard because they can choose to just focus on tank-killing after their 2 item core, but to scale into a late-game carry they need crit as well.
Compare this to mages (excluding on-hit mages), which really have a singular build "tree" - damage. Damage mixed with every imaginable stat. Tank killing, CC, mana, CDR, armor, MR, all of it. And all of it it useful the entire game. Some mages are locked ito pretty static build paths (especially DoT mages like Cassiopeia). But in general they will outdamage ADCs at all stages of the game, if they land their skillshots (mage players are far more "marksman" than ADCs, by the way).
Also compare to melee ADCs who build crit - Tryndamere and Yasuo. Both have built in crit, mobility, lifesteal/shield, and some other powerful defensive mechanic. A lot of this stuff, I can see being necessary for their balance because they're melee. But why, beyond this, do they need free crit chance? The reason, I think, is that building crit chance is too shitty and unattractive, so they must be specially enticed to built it.
Besides the fact that having RNG in this game is stupid, if crit is going to remain, then ADCs should have some more flexible build path to get there, because having to build 3 items focused purely on damage is stale. And unfortunately this isn't going to chance with the crit item rework.
