Maybe, maybe not. I've still got 2x machines running old 7950s which were mined with for over a year and they're running perfectly fine.

Gaming and general computing is such a low intensity activity - if they didn't break while they were mining I figure that they're comprehensively stress tested at this point and they'll run til they're too old to be useful.

An open-air case running a GPU (sometimes undervolted) at a constant load versus a "gamer" GPU going through heat cycles (and sometimes overclocked)?

I'd take a mining rig GPU for a discount.

The biggest problem that mining GPUs tend to all suffer from is fan failure. I'm speaking from experience.

Replacing a video card's fan is certainly doable, but it isn't nearly as easy as replacing a CPU or case fan (due to the fact that so many GPUs have custom fan setups).

Excellent point, I had forgotten about this.

Some of the GPUs (like Sapphire's) have fans which are easier to replace. These are not the norm. Also, being easy to replace doesn't mean they are easy to acquire.