But before I reel of topic more.... With multi attack they should just overwrite a single attack, not all of them. Each of the moves (grapple, shove, knock prone) specify that they replace a single attack. Making is so that it replaces all of yyour attacks is a MASSIVE nerf to martial classes.
The problem is that some special attacks in BG3 are MUCH more powerful than tabletop single-attack-equivalent-moves. So allowing you to replace a single attack with (some) special attacks is a MASSIVE buff to martial classes. Especially if combined with the ability to freely cycle through weapons (still in the game?), which could allow multiple special attacks in a single turn. As a reminder, certain weapon special attacks CAUSE THE ENEMY TO LOSE THEIR ACTION FOR MULTIPLE TURNS (again, still in the game?). Some others deal damage AND result in the target having lowered AC AND disadvantage on STs.
The Barbarian abilities in particular are all much more powerful than a standard attack. Movement+Damage+Prone ability, Multiple-target damage+Prone ability, multiple-target damage+bleed ability.
As long as a special weapon attack has roughly equal power to a normal attack, sure it can and should replace a single attack. But that is not the current case in BG3.
Fyi I'm getting my info on special weapon attacks from
here, since the BG3 wiki doesn't have descriptions for them.
