A party of up to six is the correct decision. Less than that and the game doesn't feel like Baldur's Gate. Solo the game, take only a few companions, or have a full party. That's how the original games worked. Leave it up to the individual and stop thinking that because a party size of four is enough for you that it is OK for everyone else.
And why are we pretending like the game is balanced as is in EA? Do people really think this is how the final game will be? That Larian won't change anything? That there won't be any difficulty sliders? That we can't opt into ways to make the game more or less challenging based on player preference? That's just crazy. It's also stupid to complain about how other people play the game not working with your play style or rudely pretending like you know the financials of these asks and that us peasants posting in the Suggestions & Feedback forum are too dumb to know what we want. Let people play the way they want. D&D is about giving people the power to create stories how they want. That's why the ruleset is so flexible and allows for homebrew (mods for tabletop).
Although if it ends up being that party is fixed after Act 1, then I'll really lose the desire to play this game. Party experimentation is KEY to Baldur's Gate. If you were stuck with the same party for the whole Bhaalspawn saga, then that would suck in a very major way and the pocket plane would be a very sad place.
I sincerely doubt Larian will lock us into a single party set up this go around. The relationships, the Camp, everyone staying equal level, it really doesn't lead me to believe we won't be able to change people out as long as we haven't run everyone else off with our decisions lol