ARAM is Now About Who can CC the Enemy ADC Better
There's no reason I should be able to 1v3 a Nasus, Shen, and Vel'Koz with Free Week Jhin in ARAM.
ADC has become so strong in ARAM with the recent changes to runes, the mode has basically degraded into whomever is better and CC'ing the ADC. If you don't have an ADC, then you better hope the enemy team doesn't either. If your enemy has an ADC and your team doesn't, you better hope that there's a DC/AFK on the enemy side.
Even the worst ADC can dominate. The bottom level skill floor for ADC is way up right now. You don't have to protect your carry anymore, you just dive the enemy carry. I've played Galio just diving the enemy Tristana while the Enemy Olaf tried to solo our Caitlyn, and he lost everytime. The only factor in the ebb and flow of the ARAM game was whomever could CC and distract the enemy ADC longer.
No other champion is as impactful. ADC have Sustain, Damage, Self-Peel, and slows. They don't even need a team anymore.