What is the life expectancy / MTBF of a 60 gal. water heater?
I have never heard the term "mean time between failure" used for water heaters. Love it.  I can tell you, that on my inspection reports, I advise that the average expected life span of an electric water heater is 10 to 15 years.  If you have very hard water, don't drain it annually to remove sludge, or check/replace the sacrificial rods every 3 yrs, you may not see the maximum life capability. I have seen them last up to 20 years, but that is rare.