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?. Even working temperature of CPU depend on each generation. No air-cooled 1080ti will run @70 C in any demanding game. You can argue that's only for short sessions, but the world is full of crazy gamers
miners. Many of them run in SLI, where the card on the top will reach 85C easily.
I think the lifetime depends on the card quality. I put my money in a hight quality board, like these of FTW3/Srix/Lighting/etc, running at 75C 24/365 instead of a cheap card running at 65C 24/365.
All this discussion is a bit silly. As long as the card is under warranty it's a manufacturer's problem, not our business, to make sure that the card is not fried under extreme conditions while the warranty is active.