A mortar with horrible auto-fire would be horrible against everything that is not heavy support weapon spam. Try to play against someone going 3 conscripts 1 maxim and see just how well your mortar does if you tell it to hold fire and only use the barrage. You will barely hit anything.
Your missing whats being said here
Auto attack is not being changed into ambulance levels of AI, the proposal is JUST to reduce the damage enough that it can insta Gib squads, its not going to become useless by ANYMEANS just less lethal unless told to go hot.
AGAIN think of auto attack as a squad getting shot at by a Gren squad lmg (taking bits of damage possible model snipe if rng says so) think of barrage as the rifle nade (you told it to do something and it damn well will do it/kill potential increased)
Its not "auto attack wont do anything now you can ONLY barrage" its auto attack wont wipe full health squads but barrage can, also barrage gets cooldown/rof buff.
Auto attack would remain supporting fire, but not literally game changing in an instant without any input from the player