Mages were the top strategy before the patch by far, and everyone expected them (especially Keeper) to get nerfed. That said, the changes are a bit different than the straight damage nerfs everyone expected:

The Mage magic resistance reduction bonus isn't nerfed, (6) Mages is still devastating

Keeper's nuke damage isn't nerfed, it will still wipe the board

This shows an interesting design philosophy - instead of making mages weaker, they want to make them counterable:

Keeper is squishier and requires more time to generate the 150 mana he needs to cast Illuminate.

Pipe is an item you can use to protect some of your army from the initial nukes.

Pocket Sand (the global item of Assassins) now reduces mana gained from damage, which means Assassins with Pocket Sand can possibly burst the Mage backlines before the Mages are able to cast their spells.

That said, we don't believe this is enough.

Dota Underlrods is a game in which you build your own strategy first and counter the strategies of your opponents second. You cannot go into the game with the mindset that you're going to counter Mages right from the start - there are 8 players in the game, it's fairly likely that some of the stronger players in the lobby will not be playing Mages. If you are trying to build a Mage-counter strategy from round 1, you could lose from other strategies before you get the chance to face a Mage for the 1st place in the lobby - the scenario for which you've theoretically been preparing all game.

In an Auto-Battler it's very important that all strategies are equally viable. This way you'll have the freedom to use the rolls that the shop is giving you creatively to their maximum potential. Even if Mages are easier to counter in the new patch, it's bad if the metagame revolves around a single strategy choice: