I totally agree, i can see so many Ks without any turrets left. So they are simply huge meat shields flying around without any threat. Once i even saw an I with near no turrets left. So never underastimate the attack on turrets.You are only right in as much as that individual fighters in general (except for shield ignoring weapon loadouts - e.g. Burst Ray) can not damage the hull/engines/weapons without taking out the hull shield generators. However, regardless of loadout fighters CAN knock out shield generators, disable weapons/engines, or deploy missiles/torpedoes.
Fighters should never be under estimated in terms of level of threat they pose, (with a few notable exceptions) individually they are a mere annoyance but in numbers they can still be a threat... remember the main axiom behind combat in the X-Series - "If you think you need 1 ship, bring 10".
The only thing which is really useless are defense drones, they are just too weak to deal any threat to a L ship. 10 fighters can be a threat, killing turrets engines and stuff. Not killing your ship, but making it useless. Ofc it depends on the loadout of the fighters.
So i think that the balance is fine atm. And using M ships to defend your big ships should always be done.