This is an issue which shows up on these boards fairly frequently, and the answer is that the GM is doing it wrong. As 5E so succinctly summarizes, the DM's job is to describe the world, role-play the NPCs, and adjudicate uncertainty in action resolution. It is not the GM's job to challenge the players.
If one PC actually manages to put a reliable defense together, then you should respect the player's agency in making those decision. Assuming the system is reasonably designed*, choosing to have a reliable defense means sacrificing other areas of competence. They probably have low damage, and/or poor saving throws; and the rest of the party is still vulnerable to attacks, regardless. The only way that a high AC will ruin a game, is if the GM decides to derail things in response to it.
If a level 10 monster can't hit a level 10 tank, and your response as the GM is to throw in level 20 monsters, then that's entirely on you. The alternative - where you stick to level 10 monsters, and that one character is never really threatened by weapon attacks - is perfectly playable. The danger is still there for all of the other characters, and the one tank is handicapped by their inability to do anything aside from staying alive. It's fine, really.
*If the game is not reasonably designed, then it's probably best to find a new game.