you forget other parts on the card that cant handle these temp for a long time without being affect. The cards are built to run @70C, its a fact. Now if you want to go ahead and run them @85C its your business.
Where this 'fact' comes from?.
after 70C you get a significant lifespan drop , even mining softwares change the color from green to yellow as soon as a card reach 70C
Well the science is settled then. The miner changed the color so 70c is it.