Some ideas for solving the ADC problem

Here are my two cents:

ADC item path is way too cheap. If you won't nerf their scaling, you need to increase their itemization costs.

ADC scaling is way too high. Late-game, all that matters is whether or not the ADC can slaughter everyone in a quick team fight. Game revolves around ADC so much that playing other roles makes you feel worthless.

In my opinion, we need to nerf late-game scaling of ADC's. Take them down from being the only thing that matters to being a class of champions that clearly have their niche set out for them (DPS from a range). Having an ADC shouldn't be a requirement to having a good team comp. ADC champs should be optional, and picked when it makes sense in the draft.

There are plenty of ways to nerf ADC late-game scaling

bonus AD ratios
the range itself
cooldowns
movement speed
base stats

I think if you keep the ADC's range and damage, they need to be SUPER fragile and vulnerable, so cut the base stats (armor and HP) and cut the movement speed

If you attack the range, you'd be removing the ADC's niche in a way, but that might be healthy for the game though.

Cutting the damage is a worthy option, if you cut the damage enough, the range and base stats could still be balanced. Base damage and bonus ratios could both be targeted, but if you're only tweaking one of those, you need to really gut it.