the end of season 5 was the best state the game had been in probably ever.
it was the time just when juggernauts were balanced out after skarn and mordes initial overtuned releases. around that time i noticed that you could be the carry of the game wherever and whatever you played.
if your bot lane sucked and you were coming out ahead of the lane phase as the bruiser on top lane, you could take over and carry your team. if the marksman down there won, he could carry just aswell. same for a lot of mid laners. it was really only a lot of mages who have been lacking at that point in time. that could have been fixed if it was properly addressed.
the turning point where the game really turned into a shitshow was the marksman update (which uneccessarily buffed the hell out of them) and gobal mechanics changes around the starts of season 6 and 7 which ended the clearly drawn out games with defined early-, mid- and lategames and turned the game into a snowbally clown fiesta.
increased death timers, split pushing minions, global gold changes, fucking turret first blood. this is where riot really went wrong and killed the game that we loved, in their persuit of faster and more casual games.
trying to bandaid the resulting snowball with catch up xp, and early/midgame buff of lategame scaling champions made everythign worse. half assed reworks to assassins and tanks that were way too big of a scope played a minor role in this, but still added on top of the terrible global mechanics changes.
in fact all the class updates after the juggernaut update which only affected 4 champions, were way too big of a scope for the balance team to handle.