In a battle, I discovered something shocking: Destroyers are a better value in a naval battle, IPC for IPC.
Using the calculator I found out that 80 destroyers (640 IPCs) can beat 20 battleships and 20 cruisers (640 IPCs). In addition to that destroyers can do a lot battleships and cruisers can’t do and can be in more places at once.
Destroyers aren’t king however:
Battleships and cruisers can bombard territories which may be the margin in amphibious assaults.
Unless you’re the US, you probably don’t have enough industrial capacity in your complexes to build so many destroyers.
And finally, the margin is so small that it takes 80 destroyers for the advantage to prove decisive. No power can build that many destroyers so this is purely theoretical anyway (unless you quadruple the IPC values of all territories).