I've played the game over a decade, and was a former Assassin main who's played every role for a significant amount of time, and I'm convinced that the reason why ADC is so strong in high elo rn is because Assassins and Bruisers are generally weak, or have too much counterplay for a number of reasons.

1) Tanks are strong right now. ADCs are natural counters to tanks, while tanks destroy Assassins. If the enemy team takes Assassins, your team takes two tanks and just wins the game. Bruisers and ADC are the natural response to tanks, but because anti-tank options are bad right now, Tanks will always have value even if "countered", so it doesn't make sense not to pick tanks.

2) Tank supports and enchanters are strong right now. Both types of supports naturally are extremely strong against Assassins, the former because of the above, the latter because of their CC and ability to peel Asassins and Bruisers.

3) Bruisers are weak right now. Their items don't seem to have very high impact, and Bruiser identity is being shifted towards high healing and medium damage, but although their class is supposed to be strong against any role including tanks, the insane amounts of CC in the game right now coupled with the removal of various tenacity options means that Bruisers are easily shut down and killed before they can get their damage and healing off.

With Assassins and Bruisers out of the meta, there aren't good options to deal with ADC, and there's not really a good reason to pick your traditional mages. Mages are still strong, I'm convinced they're actually overtuned, but the strength of tanks right now means you can only really play champions with good DPS, and that means only mages like Zyra Brand, and other AP champs like Lillia who have access to those strong DPS burn options are picked, and happen to be picked Jungle, while your traditional burst mages like Syndra Viktor don't actually have enough consistent DPS to be strong right now. The annoying thing is that Mages are naturally strong against bruisers since they have the burst and CC to explode a Bruiser before they can get enough damage or heal.

tl;dr Tanks and Supports are too strong right now, plus removal of tenacity, leading to a perfect storm where Bruisers are weak, Mages can't reliably zone control enemy frontline due to lack of consistent damage, Assassins are weak due to tank meta and heavy peel, and the only viable AP champions being farm-heavy AP burn champions.

This creates the perfect environment for ADC to thrive in any role NOT because they're strong, but their counters are lacking: Farm junglers apply less pressure to weak ADC lanes, and ADC naturally counters the strongest role in the game right now.

Addendum:

If we add SOME tenacity options back into the game, or buff currently existing ones, and on top of that nerf tank/support damage overall, we will create a scenario where people are actually incentivized not to just bring Warmog tanks every game, which will incentivize more bruiser/Assassin play, which will incentivize more Mage play, and lead to a less solo ADC friendly meta. If we do this, we could literally just buff ADC items and it would be fine, because our damage isn't the problem, the problem is how safe it is for us to do this damage, OR that when people DO bring our counters, they get destroyed by tanks who can just 1v9