On Fri, 07 Aug 2015 18:55:14 -0400, micky
wrote:

Because there has been no detectable crud in the water that comes out of
the sink that I touch with my hands, and if there were crud in the bath
water, I think I'd notice it.

The stuff that accumulates in the water heater is dissolved calcium in
the water. YMMV depending on what is in the water to start with. When
I was in the north east our water was very good and water heaters
lasted forever. Here in Florida, they don't last that long. Ten years
is typical.