There is one, and ONLY one up side to mana. That you don't feel like you're wasting gold when you buy an item that has mana attached to it along with stats you want. Mana is a fine mechanic if used to gate a champion from over-using strong abilities. Imagine Swain without mana. TERRIFYING.
But yeah, more and more champs are manaless, and a few of them have REALLY short cooldowns as well. Between that and the push towards mobility, this game is turning less into a strategy game and more into Call of Duty with every new champ and patch. No need to learn proper positioning when you're able to jump out of trouble with ease. It's making skillshot champs pretty bad too. I've noticed a LOT of the strong champs right now have powerful targeted abilities or ones that are hard to mess up because they have large areas of effect. Why risk missing a skillshot and doing nothing when you can pick someone who is reliable AND just as strong as the skillshot champ? Hell, most of the skillshot champs are squishy, making them even more of a risk. Go mid vs a Zed as Lux or Ziggs and you're gonna have a bad time. Just trying to poke him will run you oom, and if you mess up he's on you, because there's no need for him to conserve resources at all. Even if he's kinda low on health FROM the poke, he can blow you up and pop back to safety with ease.
I don't know why the push is towards resourcless champs, spammy champs and hyper-mobile champs... but it's there. And it's really invalidating a lot of the well designed champs in the game in favor of the poorly made "no counterplay" point and click ones that they go on about not liking.