Now, show me your heavy armor characters doing the things a DEX tank does.
Brother I've been spamming these fora with screenshots for years. If you're beating DCs by forty what do you need a tank for in the first place? Solvable game is solvable. I like to give the devs some help keeping it nontrivial where I can. Hindering class level progression to stack unnecessary AC is a way to do that I guess, but not a particularly interesting one. If you simply use the items slots and companions you have in the roles they are designed to play you don't have to do that. Most players would prefer to play the game that way but were misled into believing they had to all use the same frankenstein's monster. You simply don't.
You need to be more precise with your argumentation. If your argument is: "Heavy armor tanks are superior to DEX-based + Archmage Armor ones", then you are wrong. However, if your argument is: "You don't need a 20-dip creatura wearing pajamas to tank unfair", then you are not wrong.
Of course they are.
Your picture isn't a DEX-tank, it's nerfing the progression on a class that is already tortuously slow to come into its own to tack on a splash you wouldn't even need if you weren't trying to tank with a class that isn't set up to do so until later in the game (if at all).
Heavy tanks don't have to worry about flat-footed or being ambushed without buffs up and get to use those two item slots you're not using for significant bonuses that grow throughout the game aside from AC. With Heavy Armor Focus Mythic now a thing it isn't even close.
You have to blow Feats on Weapon Finesse and Mythic Finesse *in addition* to Archmage just to get back to where STR-Heavy already is. And no that doesn't require Smite. On Unfair Pal uses its Smites for Marks usually but can top off as needed lvls 6-11ish.
My court poet build is CourtPoet16/Loremaster3/ScaledFist1. The two secrets I've chosen are Wizard spells Mage Armor and Shield. Shield isn't strictly needed, because you can cast it from scrolls, but I took it for convenience's sake. Court poet gets most of his insightful contemplation buff online at level 8 which is a +4 bonus. To get an additional +2 you have to go through 8 more levels which I leave for after leveling loremaster. So no, I'm not really losing out on anything by not going for CourtPoet16 without dipping loremaster, because by the time you get that extra +2 CHA/INT bonus, it won't matter all that much. However, what I am gaining is CHA->AC conversion without reliance on smites... which I can also get by not getting the Shield spell and resigning myself to using scrolls, but gaining Smite Evil instead on a CourtPoet16/ScaledFist1/Loremaster1/Paladin2 build for insane AC stacking since you convert smite evil deflection bonus into sacred via Aroden's wrath. I don't need this, because I'm perfectly tanky enough and have more than enough damage without needing smites, but scribing scrolls is a real pain.
Court Poets don't have to worry about flat-footed since they get improved uncanny dodge.
Mythic Finesse is not needed when a rapier + fencing grace will more than suffice. The build is not feat hungry so I have enough feats for all the essentials.
Being DEX-based means the character is better at winning initiative than any STR-based character. Higher initiative provides not only a defensive benefit, but also an offensive one as it allows to mythic charge into flat-footed enemies via vulpine pounce at the start of combat - something that a STR-based character can't do well on unfair due to lack of initiative.
And yes, this IS a DEX-based character, because unlike a heavy armor wearing STR-based character, nothing prevents you from stacking extra AC from mental stats without relying on single target smites.
And you forgot one more thing: it's not even a dedicated tank, but a buffer who provides immensely powerful INT/CHA buff to the whole party while tanking unfair from start to finish.
Heavy armor tanks are irrelevant, worthless garbage that serves no purpose in the party, because they have to be specialized tanks like TSS or rely on smites which are not heavy-armor specific and can be used by DEX-based characters as well.