RDM could be a tank fairly easy, to be completely honest, as it was a tank of sorts in the original Final Fantasy. Red Mage was definitely one of the classes you put at the top of the party list to draw most of the attacks.

As for armor, a RDM would be wearing the same gear that LNC/DRG gets, which, if anyone had paid attention to while they were leveling, is actually usable by the three tanks as well, as tank gear. Light Armor tanks have been a thing in many an MMO, based on tank stances giving them additional defense boosts to the gear they are wearing. Taken to the extremes, you have tank classes like SWTOR's Assassin tank. It wore cloth armor, but boosted their defensive stat massively upon putting on their tank stance.

Therefore, the arguments of "RDM won't work as tank because reasons!" are invalid. It very well could work as a tank, and still be a classic RDM, because you literally cannot get more classic than the very first appearance of a class, no matter what variations later installments decided to roll with. It's why DRK as tank isn't completely out of left field, because Cecil's role as DRK was to tank and take hits, long before he became a PLD. Even FFIII's DRK was rather bulky, though the Remake fixed their lower VIT stat to something more reasonable.

The point is, RDM is a special class. We all know it. It's part of why we all love it. No matter which way they go with our beloved RDM, even if we don't personally like it, it's not going to be something that a RDM has never done before. They've tanked. They've healed. They've DPSed. They've supported parties. The iconic Red Mage is a jack of all trades. It does everything. Whether the FFXIV crew focuses on making RDM do everything, or gives it an actual focus, it's still going to be a RDM. Might be a different flavor than what we were expecting/wanting, but that doesn't mean they've "ruined" RDM. Just means they went with something they felt embodied XIV's RDM.