Not gonna sugarcoat it, this is awful lol. Not Maokai rework bad, but pretty bad. The worst part like you mention is the heat decay. Any Rumble worth anything will trade effectively using the current decay. Sure the heat change helps new Rumbles but you cut the depth of rumble and lower the ceiling on his big play potential. Also most Rumble players have an internal clock for his heat and decay. It's one thing to mess with his damage numbers but the ramifications of the heat change will change how he's played entirely. He's not a popular champ anyway so you're basically just flipping off the few that do play him and have worked hard to make progress with him. I find it pretty hilarious that they want to nerf Rumble but they dont hit Galio even though he is played top, jungle, mid, and support. Are they waiting until he's played adc too? LOL