I agree with your concept that the game was healthier for ADCs back then, it definitely was. Back then games still on average ended around 30-35 mins, which is slightly longer than what it is now but by no means was averaging 45+ min games the usual. If it was that for whatever reason I always managed to secure wins a lot quicker than most I guess.
Imo a lot of the reason why the game felt healthier back then is roles had set identities and now Riot is trying to make this awkward "try any champ, anywhere" bs. ADCs were meant to do damage that ramped up over time (generally speaking), tanks were actually that, they were meant to act as the frontline, etc. Now the game is just at an awkward state where it's having an identity crisis. Then there's what they did to the turrets where over time with changes to the turrets there was less and less risk to diving someone under their turret.
People don't like my post because I said damage isn't anywhere near as stupid as it used to be, which is still true. The main difference between now and back then is the game punishes you more for mistakes early on which has created an environment where champs snowball all the time now. It isn't common having games anymore where multiple champions have 80-100k damage because the game was actually decided 18 mins in but a team didn't know to close so they just awkwardly keep farming the enemy team for another 20 mins or so.
I don't think the game is in a healthy state but I don't think going back to how S4-S6 is going to fix anything, people seem to forget there were champions that seemed outright broken due to items, runes (RIP), and/or skill trees back then. Biggest one coming to mind was sated devourer. I do agree the game needs to pull what was good about the past seasons and try to push the game back in that direction. I just think a lot of people have a rosy retrospection over League.