Adding my voice to this thread, a 4 man party feels so much more restricted when choosing companions so you always have: 1 Melee/Tank, 1 Support/Healer, 1 Rogue (stealth and lockpick) and to wrap up a Magic Caster (which will usually be a wizard because of extended spell list, why would you take a warlock over a Wizard in DnD late game???).
D&D also has 12 Vanilla classes (not counting Artificers) which means being able to have only 4 of those 12 (33%) you won't get as much versatility or adaptability if you cold have a 6 man party (50% of those classes could be in the party). By the way, how are you going to balance a party in the future when more classes comes around like Sorcerers, Bards, Monks, Barbarians and Paladins? Who are you gonna cut off to add someone new and still feel like you have some balance?
In BG1 and BG2 I always hated to ask a companion to leave so I could add a new one, and only did so if they were someone I liked better or had an amazing companion quest I had to do before getting my "dream party" back. I never thought of those companions as disposables tools to do a job and for me Imoen and Misc would always be in the group no matter what, so at least 3 spots on those 6 man parties were already locked on (counting my OC as well).
Therefore a 6 man party seems much more immersive and fun as well as adaptable than going back and forth picking a mule to do a job and then dumping it back to camp when said job is done, which looks like where BG3 is going since you can ask a companion to go back to your camp and just collect dust until you need them for a job or to further down their quest.
I agree, when playing the original BG games I would decide on who was going to be in my team before I start and they would be the only people I recruit, with the only real exception been early when I'd recruit Monteron and Xzar to fight the mage at the Friendly Arm Inn or Yoshimo in the second just for the opening dungeon.
When playing BG3 I probaly won't recruit anyone who wont fit in my party since to me it's a little immersion breaking, "right you two sit here clean the camp and make dinner while the four of us go fight this dragon and then clear out thirty bandits in their camp"