Why don't you list some of these monsters with counterspell we can organically insert into basically every encounter?
You are the DM, you make or change monster stats at will, that is part of "planning".
Your party is fighting 10 enemies, give every one four 3rd slots and counterspell.
If for some reason you feel a compelling need to only play monsters published by WOTC, put a mage in every combat for every PC who has shield.
It really is not that hard.
This feels like the theoretical Anti-Magic field caster supremacists wave around like the zero to one time a campaign it appears somehow balances things.
That is certainly another option. See it is pathetically easy to mitigate shield if you plan for it!