I was under the impression that the "co-insurance clause" doesn't care what "survived"
I was always led to believe that the co-insurance clause "cared" whether you carried sufficient coverage based on the value of the insured property "as a whole"

So.. given tpcook's example of a $625 home carrying $500 coverage
Let's assume the cistern and slab is worth $150 of the $625... damage (and let's just assume rebuilding) costs are $625-150=$475
.... I felt... say, under 80% coinsurance... you were good and covered (minus deductibles)

Your concept of "what survived" versus the "co-insurance clause" has me confused - as it seems to be separate issues
... but, I am not in the insurance business