Recently fans have been clamoring about the power creep League of Legends has been experiencing. While true, in their quest to seek a solution, Riot developers have realized that merely reducing it solves some problems, while creating others.
In the recent Gameplay Thoughts post, via the comments, Riot Games developers have revealed that they've been playtesting the game with a 20% reduction to all stats:
"We have recently playtested with all damage from champions to champions reduced by 20%, and all healing and shielding reduced by 20%."
With this, late-game fights and especially team fights become more legible and engaging, with better structure. Tanks become very overpowered, and healing becomes much more powerful - even at -20% healing, as each point of healing is much more valuable than on the live game.
Burst mages, especially ones dependent on a single rotation - Lux is a prime example here, quickly become borderline useless and it isn't clear how to buff them without undoing the reduction in damage output.
Assassins migrate to pure glass-cannon builds, where they should belong, but the game then feels even burstier as a result. So, in conclusion, should the power creep the game is currently facing be reverted, to an extent, all we would get is overpowered tanks, useless mages, powerful healers, and burst assassins. Meaning that the game would slow down a notch, but still remain imbalanced, just in a different way.
Perhaps the only ones to benefit from this would be the ADCs, who would shine in bringing down tanks others cannot, and could be protected from the glass-cannons the assassins would and should be.