Baron - If fighters can no longer attack subs, then logically they should no longer attack tanks, BBs, CAs, etc - anything that can’t be destroyed by mg fire alone. That could be too complicated.
This is an abstracted logical reasoning.
From historical realism POV, maybe I can try to find why no single Fg was credited any U-boat wrecking.
1- U-boat armor is probably thick and made to support underwater pressure.
2- When diving underwater, U-boat get some additional water around the hull. This water is a physical barrier against machine gun bullets: a few inches slow down the pace to armless speed. (See this on Mythbusters TV show.)
3- On the contrary, a detonating bomb can make more damage under water due to an increase shockwave effect.
So, Avenger torpedo bomber can damage a Sub above and underwater while F4F-Wildcats have probably a very narrow opportunity window to make significant damage on a Subs above water.
This is how I rationalize the historical facts on U-boats warfare.
From a game play POV, I played with a cheaper and weaker Fg A1 First strike D2 M4 Cost 6, hit plane first if any, can never hit Subs.
It’s not more or less complicated than subs which cannot hit plane.
It was necessary from a balance POV vs Subs, Destroyers and TcB.
My Fg was same cost as Subs 6 IPCs, have almost the same combat value of a destroyer (A2 D2 M2 C8, ASV) for a lesser cost.
And TcB A3-4 D3-4 M4 Cost 9 have a specific capabilities against Subs while my Fg have a specific ability against plane.
This make both units useful in carrier operations.
In your perspective, a 8 IPCs Fgs with A3 D4 M4 is very powerful compared to any Subs (Fgs are immune against them), Destroyers (same cost almost double combat value against them) and Cruiser (A3 D3 C12, Fg completely outmatch them).
This creates very little interest for the others sea units because Fgs can do Sea and Earth operations with more range.
So, if it has historical grounds, and it is needed for game balance to counter-weight for the high combat value, the Fgs never hits Subs can works to keep 8-10-12 incrementation cost.
See this as a different and new perspective, that you are introducing a real Fg unit (A3 D4 C8 with Machine guns only) in the roster, while the old OOB Fg (A3 D4 C10 was doing Fg-bomber, torpedoes and dive bombers job, since it was the only unit able to be put on carriers) keep the same cost but received a change A3 becomes A4 and D4 become D3.