Subs would be too powerful at only $6 if they were allowed to choose when to soak hits or when to crash dive.
I have 2 words for the original poster:
Combined Arms
eg 6 ftr @ $60 vs 2 ftr 1 CV 3 DD @$58 is only 51% to the attacker
Make it an even $70 each and you get an extra ftr for the attacker and a Cru for the defender
Attacker wins drops below 50%
Battleships are definitely a poor option in a naval arms race, since statistically a DD + Cru always performs better in stacks.
However the coastal shot @4 and the ability to soak hits in the little battles (like when your opponent block with a DD, you can soak that hit he scores 1/3) mean that BB’s have their purposes.
I rarely see a BB built though, usually one on either UK1 or UK2. Occasionally the USA will build one if going KGF.