The DM has enough on their plate without having to plan every encounter around a single overtuned spell (see also silvery barbs). "Git Gud" is insulting advice in a video game, it's equally insulting here.
God forbid you actually run a dungeon in Dungeons and Dragons, which often feature small quarters and 1-2 opponents, which are easily blocked in and made irrelevant by a 1st level spell.
Then don't bother planning every encounter around it?
I'm reminded of when the Tunnel Fighter UA was being commented on, and people were FURIOUS about the idea that a character with that fighting style, and polearm master and sentinel could hold off a 15 ft hallway against any possible number of melee opponents! Which, at the time I pointed out... yeah, that's perfectly fine. That is what should happen when a character devotes a bunch of resources to specific gimmick.
Whether it is because they are an Eldritch Knight who grabbed the Defensive Fighting Style, is using sword and board, took warcaster to cast with a weapon, and uses the Shield Spell, or if it is a Bladesinger with high dex and high Int, who casts mage armor, activates blade song and then uses the Shield spell.... yes, they devoted a lot of resources into getting a very high AC. And at low levels they can't use it very often. And at high levels, enemies will still be capable of hitting them. So the "problem" is two builds at mid-levels... and yeah, I don't see that as much of a problem when the enemies can get past that high AC with different plans and strategies if you truly, truly need to cause hp damage to the target.
I've played with Shield builds. I've played with people who have Silvery Barbs, my current party has TWO characters with Silvery Barbs and we've still nearly TPK'd multiple times. Actually, the spell our DM hates the most (though Silvery Barbs is second) is the Wizard's Vortext Warp spell, something that essentially never gets mentioned as being game changing, but has been absolutely pivotal in multiple situations.
Can it be frustrating? Sure, it can. Just as it can be frustrating to have a Battlemaster Fighter/Oath of Vengeance paladin drop an action surge, Oath of Enmity, multiple smites with maneuver dice nova on my Boss monster and make them a greasy smear on the floor... but I don't think the correct move against that frustration would be to nerf action surge. The player built a character who was really good at one thing. They can be rewarded by being good at the thing they built themselves to be good at.