the other issue is a pure metal through hull
with no bond will not corrode.
if the cast has mixtures of metals. even unbounded you have dis similar metals in a circuit, that you can't disconnect. and needs to be bonded to a zinc to save it.
the quality and cheapness of everything made today has to be a concern for this issue. IE I doubt a brass through hull
made in china
isn't full of other metal as well.
a 30 year old through hull may have lasted 30 years unbonded. a new one might not.
same issue for engines when you have shafts / props / engine
blocks etc all connected but different metals.