Usefulness of high AC at high levels (Tier 3 and 4)

If you're looking for better optimization paths, at T3 and higher its (in order)  Spells, Magic Items, Subclass abilities, as these have the ability to rewrite game rules.  Where as raw bonuses have diminishing returns, as higher CR creatures scale up faster than you can stack bonuses and sustain them.    In particular, Spells like Haste that can add to a group's action economy is only outclassed by magic items that can add to a groups action economy for even less of a resource cost.   Sword of Paruns lets an ally trade a reaction to copy what you're doing, which otherwise costs an action on their turn.   There are few magic items that have some degree of, or a full casting of, Haste as an item property.  Items that mimic Counter Spell or Dispel magic, Vopral Sword is an instakill on a Nat20 for most non-boss level creatures, the Bag of Holding placed in a Bag of Holding, theres a pendant that gives you 25 THP when something dies nearby (and when it runs out of charges, it summons a dozen skeletons), a rare Amulet that tied to a Shield Guardian (and coopt an existing guardian), multiple items that give you a fly speed, Vicious weapons are significant DPR boost against most enemies (and by the time that stops being effective, you're better off finding a save its weak against), Staves that cast spells (a lot of powerful ones too), Plate armor thats basically a Space Suit, or the Mizzium Apparatus for the sophisticated Wizard who feels a prepared spell list is but a gentle suggestion.  

As much as one can argue this is all subject to DM fiat, so is everything else in the game.  But if I had a choice between an Very Rate item that gives +3 to spells, or a Very Rare Crown that lets me maintain an extra 12 undead skeletons and doubles their HP.......  I know which one is the more fun option.