Iâ€™m not sure I understand your issue. BG3 adds plenty of characters beyond the four PC party limit. Youâ€™ve got temporary companions in summon slots. Also, thereâ€™s Halsin, Volo and other camp dwellers who you canâ€™t control. They are as close as the game has to DMPCs.
Do you wish the game added allies that followed you around in the world and participated in combat without player input? Or is your issue with the basic premice that DnD is balanced around 4 PC parties when source books keep adding to the party beyond that limit?
No. My point is that it has been argued that party of 4 limit is based on standard D&D 5e campaign builds. They are standardly designed for 3-5 players. But, my point - not issue, just making a point - is that in most campaigns the story gives PCs the ability to get more NPC companions that join your party. This means that, in truth, most encounters in said campaigns are designed with a larger overall party in mind.
In BG3, all the characters but the MC are NPCs. Thus, it is no different. We should be allowed to take all 5 of our NPCs with us when we adventure if we want. The argument that D&D campaigns are designed for party of 3-5 doesn't really hold water because in most campaigns parties increase with NPCs often to 6 or even up to 8 depending on number of players. And many DMs will hand out these extra NPCs for players to control during battles so that the DM isn't managing/ rolling as much.
So, again, I'd like to point out that when I play multiplayer with 4 players, at present, I can't add a single NPC origin character to my party except during the prologue. I'm full up. I'm stuck with just a party of 4 players. No NPCs allowed except the very temp ones like Sazza or Halsin. Nevermind that I have 5 to choose from. I'm stuck with just my 4 PCs.