Know what makes Rumble cool? He is the SOLE MANA-LESS CHAMPION IN THE ENTIRE GAME THAT PAYS FOR IT!!
Anyone who tried picking Rumble up before will tell you, that Heat system of his really eats up an amount of attention that makes playing him unique compared to other champions. There are pay-offs for managing his Heat system. He is punished if he can not manage it well.
Put this in comparison to 
, 
, 
, etc. To them, they don't have mana, yes. But they literally have no resource to worry about anyways, despite "Energy" supposedly being a resource too, but the majority of them don't even know they're Energy champions due to how trivial their costs are! Even someone like 
is atleast punished if the player is careless with his abilities (his R can backfire HARD if you are careless).
But in the very end, Rumble is probably the very few, or even only, champion in the entire game where being mana-less comes with a price.
And now that you decided to give mana champions the middle-finger with your mana regen nerfs WITHOUT doing jack to all the mana-less champions in the game, the disparity between mana users and non-mana users, who fought in even grounds or was already in favour of the manaless ones might I add, further increased. And some champions already struggled with their mana BEFORE this change to begin with!
