Mana buffs and mana regen nerfs for mages are actually good for the game.

It puts priority back into blue buff and is an overall late game buff with the mana regen mastery, which restores a percentage of your MISSING MANA. In addition, mages will have trade-offs for their damage and utility, since they won't be able to indefinitely spam anymore, allowing more power to be injected into various situations for them...If Riot follows through with this direction.

Lowering the slot efficiency of a lot of the itemization though...that's just kicking mages while they're already down.

AD items in general are still over-tuned and need some pretty heavy nerfs, especially if the new speculated values on AP itemization are going through.

Mages still need help, but being able to indefinitely spam spells isn't going to help them. Mages being reliant on mana regen in that way keeps them weak in the same way Vlad and Akali were nerfed to hell for utilizing spellvamp well.