Naval surface is already being killed by heavies as is, having subs would somewhat balance that threat. Heavies will finally be made vulnerable and will require the cover of destroyers to push a certain point, DD will play an important role in searching for submersible threats.
Can’t say for suffocating modes like AB, I only play naval for the joy of EC.
Minelayers would also work nicely against both subs and heavies, in the defence of ports (early game) if their mines would just stop despawning for no reason in EC, esp when they’re not shot at or destroyed by depth charges. Minelayers have no chance to do anything when heavies have already rushed the ports, so it’s seen as a fair compromise, lay your eggs in a certain basket early game, and hope that the enemy rushes that certain port blindly.