1 : I did a lot of DPS stance tanking as a Warrior in Tera and even tanked some of the end-game bosses as a Slayer which is an only DPS class. Granted it's not the same kind of gameplay but still, tanking out of tanking stance is not totally new in MMORPG history.
2 : Each game has different rules. What is unthinkable in a game has a chance to be completely normal in another game. That's what people tend to have issues with in FFXIV. I don't think the DPS stance has only been made in order to off-tank DPS while not tanking anything. The majority of the time bosses hit like a wet noodle (in every content, even in raids) and only hit like a truck during specific and scripted moments. The tanking stance is only needed when you need to generate enmity and during those specific scripted moments where heavy tankiness is required (whether it's a tank buster or just a very damage-intensive phase). The rest of the time you can and should completely be using your DPS stance. And I believe it's working as intended. If the developpers only made DPS stances for off-tanks when not tanking anything, everything would hit much harder and the tanking stance would be required even for regular damage. I think the developpers wanted us to be able to see when and where we need more tankiness and to dish out more damage the rest of the time. Exactly the same reason why healers have Cleric Stance. This reasoning is completely unthinkable in a lot of other MMOs but here we're playing FFXIV, not other MMOs. And that's how this game has been made. Now if the developpers wanted it, they could make everything hit a lot harder thus requiring full time tanking stance. But currently, that's not how the game has been built.
With that said, if OP's idea was one day applied to FFXIV, I would quit. It would just take away what makes tanking and healing fun in this game. Because well, only focusing on tanking and healing is really fucking easy and boring in this game compared to others. The difficulty comes when you try to surpass your role and actually play your job at 100% of its capacity in a raid. And I like that. A lot. It kind of "breaks" the trinity without completely destroying it. We still need tanks to tank, healers to heal, and DPS to DPS. But tanks and healers can do good DPS too and actually participate in the boss's death, and that's great. I don't want my responsability as a tank reduced to just being a meat shield and nothing else. Why would I fight with a ridiculously big greatsword or an oversized battleaxe if it was not doing any damage ? I could as well just be standing there insulting the boss. It would take way less efforts and do the same effect.