As I've brought up in the BG3 VS WotR thread, there is an argument to be made in that a party of 6 will ultimately lead to a lot more party/build variety at the end of the day. Obviously.
I remember when PoE went from 6 party members to 5 in PoE2. I also remember that the community reception to the Druid class in particular completely shifted overnight in response. It suddenly went from a great utility/crowd control caster with a shapeshifting ability, to a class considered to do some things well but not worth the party slot compared to more specialized classes like Clerics, Wizards, and Ciphers.
I actually would not be surprised that if BG3 ultimately remains at 4 party members that the metrics will start showing some classes/companions just being completely ignored by most of the community in response. There's a reason why no one is really asking for the Monk or Sorcerer classes and companions in particular, while most people are asking for the Paladin and Bard.