Two questions come to mind:

How hard is the water there?

Have you ever partly- drained the water heater to get some of the bottom sediment out, before it turns to insulating concrete around the coils?

OK: 2\ufffd - ever hear popping sounds, as the heater warms up?
(That's usually chunks coming off the solidified coating; for most - that signals, "you waited too long".)

All other things being 'normal', and the odds against failure of two thermostats (even to '05 standards) being long.. I wot that very few ever use that drain valve.
(I include myself, though I tend to remember at least annually; so far OK after ~ 20 yrs. Not terribly 'hard' water though.)