The DM's job isn't to win fights. It's to challenge the players. If the DM wants to win a fight, they can just snap their fingers and summon 20 Tarrasques. If they want to give their players a fun, but challenging, combat encounter, they can do a mixture of solutions for a character with a really good AC (AoEs, non-damaging spells that still harm the character, environmental hazards, etc).
Absorb Elements is a reaction, so it's competing with Shield and Flash of Genius. Additionally, there are plenty of AoEs/Save-Against-Half-Damage effects that don't use elemental damage (Green Dragon breath weapons, quite a few spells, etc). Yes, a lot of them are consumable, but if combat is lasting 5 rounds at most, the consumable/recharage abilities will still get their time to shine.
Then don't. Mix in other things (monsters with high bonuses to hit, spells, other saving-throw-effects, environmental hazards, flying creatures that are out of reach of melee characters if the PC is melee, etc). I was just giving one of the main examples. My point was that high AC isn't a "I win" button by the players (DMs have a monopoly on those in D&D, after all), and my other point was that Warforged and Artificers aren't the only way to get really high AC (but you weren't addressing this point).
So? That's a buff to having high AC. High AC should have buffs, otherwise the player will feel cheated out of their abilities and work as a character-creator. However, just like there are ways to damage and debuff PCs that have high AC, there are ways to protect enemy casters that aren't solely reliant upon Opportunity Attacks (abjuration spells, flight, surrounded by guards on all sides to make it so melee characters literally cannot get up to them, etc).
High AC is great, but it's not the end all, be all of character options and optimization in D&D. The DM has literally every tool in existence to make High AC not be a game-breaker in their campaign.