Unpopular opinion, I think this change is actually a good idea.
To me, Mana management is something that is missing since Season 5. Having this change through would force players to have a better management of their ressource, which means eventually a slower game path, and a longer lane phase duration.
But Mages shouldn't be the only one to "benefit" these changes. Marksmen should also be affected by it and even harder since they rely on basic attacks more than spells as their primary source of damage, yet they are still able to poke well with it in lane.