Here's that education you were looking for:
Base mana changes approx net 3-5 spell cast worth of mana lvl 18 nerfed vs 1 spell cast worth of mana lvl 1 buffed (hint: x mana matters more when you only have 400 vs when you have 2k. Also stat growth statistic is not actually constant, but an average. Lvl 1 -> gives like 50% of the average while 17 -> 18 gives like 120%~~ of the average. So the breakpoint is muuuuch later into the game.)
Dorans ring gets 5 minion damage and mana is no longer linked to minion kills (I.e. enemy zoning you is not as bad as it was before. Also mages in non CSing roles like supp and jungle can reasonably go dorans ring first back since it's actually really freaking good that such a cost efficient item is now not locked into solo laners)
But wait there's more
Luden's gets 100 mana (~ 1 spell cast) buffed, I think GLP gets buffed too by 100 mana
Lost chapter goes from 10% mana to 20% mana restored, gains 10 AP
But wait there's more
AP buffs to Twin shadows, Spellbinder, rylai's
Minor mana regen buffs
Complete bs
Because no matter how you look at it, that is a nerf to mages and there is no compensation AND win rates for mana-mages in general aren't particularly outstanding to begin with
It's a power shift into early game mana and late game damage from early game damage (loss of amp tome in some build path, very slight since lost chapter is given AP to compensate, like 10 AP) and late game mana. Which does benefit a lot of burst mages who struggle early in mana but only need a couple of combos if not just one late game to get rid of a key target. Meanwhile mages who don't look for early ability usage but transition into ability spam are nerfed.
Not '''''no compensation'''''