The thing I don't get is how do you justify triremes upgrading to frigates, and both frigates and caravels upgrading to destroyers, yet ironclads can't upgrade to battleships? The only reason I can see is nostalgia from previous civ games that just requires that ironclads suck.
You are right. I remember that ironclads were usable in Civ IV only with the circumnavigation bonus and navigation promotions. I agree in that they should be upgradeable to battleships and able to cross ocean tiles. Maybe an ocean movement or ocean combat penalty should be enough for them.