Through hulls fail.
Sure, chainplates fail too and so do shaft seals
, toggles, rudder
quadrants, keel bolts
and waterstays. The hull
may delaminate or open a crack, the deck
joint can come apart and grp hulls built in halves can part ways. Rust never sleeps and so does electrolysis
Seriously though, a failure from a well designed, build and maintained thru hull
made of appropriate material is such a remote
possibility that 99.99% percent of naval architects resolved to employ these inherently unsafe gadgets.
I do think it is nice if the through hulls on any boat are in places where they can be immediately accessed and checked.
The only time I have seen a through hull issue was on an old home built ferrocement boat used in our marina as a water
cottage. Not a sailing boat built and ready to cross oceans.