Educate me: what is the purpose of submarines in the game? (Excluding nuke subs.)
Sure, T1 subs have some uses: raiding underwater mexes, maybe doing some damage to frigates (mostly before the opponent has T2 air/navy).
Destroyers are a strong counter to basically every sub (well, maybe not Atlantis), and torpedo launchers are a hard counter (except where the sub has a higher tech level and therefore more range).
Punishing someone who builds only cruisers? Sure, though frigates are also an effective counter.
Stealth? Yes, but (a) scout/spy planes can spot them and (b) a stealthed sub army isn't actually very useful. (Maybe it would be more useful if they could hit all T1 engies without having to surface, but not a major thing.)
I'm 1100-1400 ranked and don't play a lot of navy, but when I do, subs (especially T2 and Seraphim T3) don't seem to have a lot of use.