Here are the problems as I see them:
(New and presumably Revised) Monsters don't use Counterspell, they use Negate Spell. Therefore, players will not be seeing the benefits of this nerf, it's only there to make monsters harder to shut down. (And I use the term "shut down" liberally, because being countered is nowhere near as devastating as being actually shut down with a condition like stun.)
I know there is a contingent of people who want Counterspell nerfed on the basis that "Counterspell should be nerfed, it's too centralizing" But that ignores the fact that Counterspell has already been nerfed: Monsters use less spells now, instead relying on Magical Attacks and ability's that are not spells to do spell-like effects, and sometimes peppering in a real spell as part of the attack action or a bonus action, maybe even a Legendary action. This dramatically alters the action economy angle of Counterspell. You are no longer trading a reaction for an entire action, you are trading a reaction for a small part of a round. Meaning Monsters are already harder to stop with the Old Counterspell. And this is completely sidestepping the fact it is easier for several kinds of monsters to just shrug off a new Counterspell.
Finally, Counterspell is basically the only counterplay there is against most spells. If you are finding yourself staring down the barrel of a "Save or go play on your phone for the rest of this combat because you don't get to act" situation you are just SOL now. Sure, losing one turn on a solo monster was bad, but losing an entire combat as a player is undoubtedly worse. This is a problem that can only be solved with a massive overhaul of mechanics for basically every spell in the game that functions as control, and WotC has already implied they are not willing to perform such an extreme revision for the new rules.