Actual life expectancy of COTS power supplies
Without a HALT or QALT (highly or quality accelerated life test) approach to testing, there is no way to accurately assess the long-term reliability of a product in a short time that defines MTBF. 

The only way to prove an expected life is to test it for that duration in a nominal environmental.  One can establish a Confidence Level of a failure rate based on FITs or MTBF and time, but only time is give 100% confidence in a result.  

Stress to reduce the time and measuring margin to failure is the best way to devise a product Design Verification Test or DVT plan.

It also requires an Ongoing Reliability Test (ORT) to ensure the quality does not change.

These methods require skill to determine the type and amount of environmental level stress and Arrhenius acceleration rates.

e.g. a "constant" failure rate of  0.25% per year is an MTBF of 400 yrs  but the MTTR could be 10 yrs or &lt;1 yr if used improperly.