I wouldn't mind mages having to worry about going oom and carefully manage their resources. It would be a good balancing point for them. Some mages could have low mana costs and some high, based on their power, range and so on.
EXCEPT mid lane (and a lot of the game in general, really) is dominated by resourceless or AD champs. So as a mage I need to be INCREDIBLY careful casting spells... you know, the ONLY THING I'M GOOD AT AND HAVE BEEN PICKED FOR... or I will go oom and be utterly useless and forced to B. Meanwhile Zed is spamming his spells like mad every time they're off cooldown, and likely spamming /laugh as well. And even if he somehow misses everything or eats a silence or something, his auto attacks hurt like a bitch AND probably heal him, since he has access to lifesteal.
It's never been a balanced system. If mana-regen is high then mana isn't much of a concern and it may as well not be in the game. If it's absurdly low, no one takes mana users at all because they suffer long periods of uselessness while they're oom. Mages already have to buy mana regen to remain useful most games and CDR to cut down on said period of uselessness a bit. Why do AD champs get to build to emphasize their strengths but mages often have to build items to try to cut back on their many in-built weaknesses?
If you want to nerf mana-regen, fine. But equal the playing field so mages can compete against the non-mana users that they will often be up against. Mages either needs to have VERY powerful abilities that merit how infrequently they can be used. Mages shouldn't blow up an entire team from full health, but they shouldn't go oom in lane after one and a half rotations of spells either! How about giving mages high base mana regens that scale up at a low amount, so lategame they're occasionally going oom if they're not careful or built a bunch of mana items?