I think now that the no subs vs subs combat is not necessary at all.
Explaining: subs are considered a separate group inside the navy.
Allowing subs to attack subs still be as OOB rules.
For example, 2 subs can decide to attack another 3 subs with 2 DDs and 2 TTs in a same sea-zone.
If attacker target subs, then the defender had only to decide whether or not his 3 subs will submerge or combat.
Many will prefer to submerge, so there will be no combat.
If defender choose to fight, then it will be 2 subs@2 vs 3 subs@1, as per OOB rule.
So sometimes, but seldom, subs can sink subs as history reveal.