The Assassin Update should have came before the Mage Update
The biggest impact on the health and viability of mages is the assassin class. Nearly all assassins have very little counterplay available to them and mages are forced to share a lane with them; mages suit their item builds and playstyles pretty much entirely around assassins when they're in the game with them. Reworking the assassin class, then looking at how mages are effected and doing changes to mages accordingly would have been a far better way to go about this.
If assassins had better trade-offs and gave their opponents more of a chance to fight back, people wouldn't be so worried that the lack of mana regen would make mages unviable against them. Assassins shouldn't be outright stronger than mages in a fight, they should be equals.