The rationale behind cool-down only champs?

Does anybody know Riot's rationale behind champions that only have cool downs? That is, champs that are not constrained by mana, energy, life-per-ability or something unique like overheating (e.g. Rumble).

I used to think this was a vestige from the old league days, but there are recently developed champs that also enjoy the unencumbered nature of being manaless. Is this because they cannot use both a rage bar AND a mana bar (e.g. Gnar)? You'd think they could use a counter like Ashe's passive for Rage on Gnar or Fury on Rek'Sai.

Anyway, I find it interesting that this group of champions enjoy being disproportionately represented in the top two tiers of viable champs (depending on the list you consult). I know some may take this discussion as me whining, but I'm truly interested in what makes Riot choose whether or not a champ will have to deal with lane sustain issues. If anyone knows...