You say it's different in a million ways. I say it's different in one. In that you don't like one option, but you do like the other.
After all, it "shouldn't be the player's task to self-restrain" down to four players instead of six when the "most optimal" approach to combat is using six characters.
"The Help action brings downed characters back to 1 hp, but don't use it if you don't want" is different from "The game allows 6 characters, but only use four if you want four" in a lot of ways.
- The "Help" action being implemented as such prevents playing a certain way (e.g., a way where you can't just easily heal any downed ally, but you can still attempt to stabilize them), whereas an optional variable party size doesn't prevent playing a certain way.
- There are positives and negatives to 4-vs-6-person parties. A 6 person party offers more flexibility and additional party banter, but requires more work managing them - both UI-wise and possibly getting them all to approve of you. However, the Help Action restoring allies to 1 HP is a strict positive since enemies don't typically get Death STs and would often attacked downed/stabilized allies anyway. Not using it is a strict combat nerf.
- Ideally, the game can be balanced for all party sizes via scaled exp (or other solution), so a 6-person party wouldn't be the optimal solution. This ties into the above point; the Help Action would still be the optimal choice in many situations.
- BG3 currently allows for 1-3 player-parties. This is fine, because it is a quantitative change in playstyle AND clearly not the intended experience. Similarly, having an optional 6-person party would be equally as not-intended. Not using the Help action, however, would be going against the intended experience and would be a quantitative change, completely ignoring a mechanic instead of just having less or more of it.
You are correct that, if 6 players is strictly superior option, it shouldn't be the player's task to self-restrain down to 4 players. For this reason, the option to play with 6 should need to be manually turned on in the game settings, and not be available in-game without doing so. Having players opt-in to a broken system is much better than continually forcing players to opt-out of a broken system. It matters what the default is.