Failure rates vary greatly, and everyone's experience is their own. In any case, with lower average temperatures, one should expect less failures over time. For most of us, I think we tend to replace the hardware (upgrade, new toys, etc) before the GPUs fail definitively - but there's certainly exceptions now and then.
My "hot" 980 runs 80% fan, and the "cold" one 70% fan (both reference cooler designs), and both of them sit at 70C (+/-2) temperatures while hashing non stop. I think that 80C is perfectly ok for these cards, but if you can run them cooler, all the better.
Note that a lot of the GPU failures are caused by VRMs running too hot (most times, they are much hotter than the GPU core itself). Cards with backplates should do better in this respect, but only slightly.
Happy