The manual also indicates this, on page 147:
"
DOMINATION VICTORY
If you capture all other civs’ capitals and hang onto your own, you’ve achieved victory.
If you’ve lost your original capital, but still possess other cities, you can still win another type
of victory: culture, scientific, or religious. However, you cannot win a conquest victory until
and unless you recapture your own"
Which lends back to the reason why capitals should not be able to be razed....