Riot Logic:
Rework a champ putting 80% of his power budget in asingle spell -> Repeatedly nerf said spell until the champ's mains cry black tears.
Imho the barrels should have been balanced this way:
Mana: 0 -> 40/30/20/10/0
ArPen: 50% -> 30%/35%/40%/45%/50%
Flat Dmg: 80/90/100/110/120
This way denying a barrel or dodging them would be even more of an advantage for the GP opponent. The usual two-barrel combo would cost 80 mana, which is huge for a champ with that mana pool (perhaps too much). On the other hand the lower ArPen in the early game would help melee toplaners, who are usually gifted with higher base stats than ranged toplaners and tend to buy more defences than them.
The "high" flat dmg rank 1, though, is necessary to make the barrel poke satisfying and worth playing its mini-game. Otherwise, provided you have the guts to play GP next patch, you are forced to farm under turret for 20-25 minutes until you get at least two core items and be a shadow of your former self.