What is EA is certainly not even half of what they actually did. Since the game was to be released this year, they had to work on Act 2 before EA's launch. If they rolled out this thing in EA now and found it didn't work then people would complain that they had been lied to if it didn't come out in full. If Larian introduces the option to choose the size of the team and it is unbalanced at the same time, people will start complaining. It's safer to just not complicate the matter and let people who want to play with a non-standard team to use the mods. The game is designed for cooperation and such games usually have either 2 or 4 players. I have not come across a game that would allow more people to play. As a rule, it's hard to find more people to play so that the dates suit everyone. It makes no sense for some players to control more than one character. Another thing is that the game is also to be released on consoles.
So we don't actually know if it is going to come out 2021. A general consensus is that this game is likely going to be done sometime in 2022 with many hoping for late 2022 for the best product. Them having worked on further Acts sets nothing in stone, especially the difficulty of the game, they can easily adjust encounters by adding and subtracting enemies or changing enemy statistics, like they already have done. Tuco's playthrough and many people's observations find that a 6 player party doesn't actually break the game and in fact feels more balanced to the encounters presented to us. And nobody is complicating the matter, this is EA feedback, and a large amount of people are providing the feedback that in fact they want a 6 person party to be the standard party. I argue co-op would not actually hurt the 6 person party at all as you just distribute the extra characters. If two people are playing then both players will control 3. If three players are playing, each will control 2. And if four are playing, then 2 players can control an extra person. (And personally I would like to abandon the notion that coop must be 2 or 4, cause a 6 person coop game in BG3 actually sounds fun and akin to dnd). Also how is console releases going to affect a 6 person party in anyway? I have Icewind Dale Enhanced Edition for the PS4 and it controls perfectly fine and plays perfectly fine. i use a 6 person party in that game and in fact it actually controls much better than BG3, so honestly I don't get why BG3 being considered for consoles be even a factor here?