Most scale oriented mining rigs I've seen pack cards super close together, even if you run them at reduced frequencies thermal environment is not great. See https://us.v-cdn.net/5021640/uploads/editor/do/j86gsm9cxvl2.... for example.

Aside from a few top end models modern desktop GPUs are not designed for continuous operation at max load. When you combine than with higher temperature the lifespan of the cards is indeed reduced. The real question is how much this matters.