Well it depends on you lowering the voltage and underclocking you GPUs can make its lifespan longer if you are good with the lower hash rates however considering what you have right now its already good specially the temperatures of your cards not exceeding 70C. The only problem that is that how lucky you are with your GPUs because of the silicon lottery. I believe silicon lottery not only affects the performance of a GPU also the lifespan along with the vram and capacitors of it. Still I am happy that you turn to ETH mining wish you luck on it.