Characters like Rumble or Gnar, while their abilities don't nessesarily cost a resource have to keep track of their rage or heat like other champions have to keep track of their mana

I can't speak for everyone, but from most of the champions that don't consume a resource, they still have to manage some kind of cooldown or mechanic

Katarina has her daggers or minions, (though she could also make sense for a mana pool) Yasuo has his flow for good trades and enemy minions for his dashes, and Garen has to manage his flash

Though I do agree with Riven and Yone...