The problem here is that with mana-regen being removed from items and nerfed on the few keeping it, you will burn your mana 10% faster and then what?
The issue is that with these changes mages will not be able to sustain in lane. The fact I can cast my spells faster doesn't help me if I can't get my mana back (so I won't be spamming then anyway). So, when laning, I'll just back constantly for mana, allowing Zed/Yas/Corki/Talon to free farm gaining gold and experience on me.
And don't play the "manage your mana better" card; mages primary source of damage is their abilities. When getting harassed by the likes of Zed, you can either not CS (or get poked to hell) or use abilities to wave clear. If you use abilities to wave clear, you can't use them to harass your opponent, which means your opponent will bully you out of lane. If you decide to use your abilities on your opponent (particularly AD ones who can out sustain you with life steal) to try and not be bullied, you will ultimately lose a war of attrition and have little to show for it. No matter what you go, you end up having to back and the AD opponent free farms.