What is the average life expectancy of a DAC?

The answer is that DACs will last as long as you are satisfied with what you have (and it is still working). That could be 1 month or 30 years. YMMV.

My previous one lasted 15 years for me, and was used when I bought it.  It was a Chord DAC 64

@jaytor 
I have a similar DAC path, only with a Wyred 4 Sound DAC-2 DSD v2.2 in between my Yggy and Terminator.  I recognized the benefits of moving up the DAC chain, my experience has been better DAC's cost more.

I found the Yggy better than my CD player's DAC but mildly unsettling at times.  The W4S was smoother and more detailed with better low end but still occasionally strident.  The Terminator was a big step up but still at moments left me wanting.  The Terminator Plus with Denafrips Iris DDC using the clock out from the TP is amazing.

I could always spend more and get better now.  Unless I get another upgrade bug (can't see these coming) I think my DAC will be good for at least 4 years and probably more before DAC's change enough for me to feel the need to change.

Just curious how everyone thinks about FPGA systems like the PS Audio Direct Stream versus chip based D/A systems. Does the ability to modify the system via a software update give FPGA systems a longer significant lifetime and therefore do they warrant a higher used price?

I like the fact that my Aqua La Voce S3 is designed to be upgradeable, but I bought it for how it sounds today, not in the future.  I bought mine 2nd hand for about 60% of retail, so I wouldn't say it commands a higher than average used price. 

I'm a little disappointed that it sounds exactly like a $100 Chinese DAC, but it's big and exotic and somewhat expensive, so it has boosted my fragile self-esteem.

As others have said, no moving parts just electronic components. I have had  a Meridian Direct (Director) for years. Its moved from my main system to my shop system. I’ve dropped it numerous times and it gets full of sawdust, still has wonderful sound.