Your forgetting the sever lack of mana regen on the Mage items, meaning sure the mana pool will be bigger in effectiveness in first look, but with near no mana regen it will mean the exact mana before OOM will be less due to the rate of refill being drastically less
Also the eternity passive is absolute shit whe. You take into account that you have to take damage to get mana and when you use mana it heals for next to noting
Going through 100 mana means healing for 15 hp
Taking 100 damage from a champion is 15 mana, which isn't nessesarly bad, until you take into account the enemy isn't going to just poke you and. Leave you be
For it to heal what old version did on lvl up the Mage would need to spend 1000 mana
To get the mana the old one have per lvl up we would have to take 1000 damage
If they buffed the % a bit it might be a decent effect but currently it's trash and it would be better to just put hp and mana regen in the item