You can give your druid the criminal background and do not use a rogue, you can train your cleric in perception and survival and your ranger could stay home, you can use a half-elf sorcerer and give him the charlatan background so you do not need a diplomat, your half-elf rogue arcane trickster could be a sage and fullfill all your knowledge needs, your paladin could make a performance to rival your average bard, you can use a gith hexblade warlock and you do not need another melee fighter.
I do not say that´s a perfect, but That´s something you cannot do in previous editions. Just saying.
So I read this, and all I can think is that you are telling those of us that want 6 person party size that we should just abandon the pregenerated companions (missing out on flavorful dialog and plot), so that we can make a custom 4-person party that doesn't follow the 5e "classic" composition of Warrior/Mage/Priest/Scout, but can still fulfill those roles by shoehorning in proficiencies.
Note, that I do play D&D 5e, and game with a party of 5 other people (6 total). Saying that D&D 5e was built around a party size of 4 to me is just disingenuous. AD&D (the edition that BG 1-2 was based on) was built around a 4-person party (yet the game had a 6-person party). D&D 3.X (the edition that IWD 2 & NWN was based on) was built around a 4-person party (yet those games had a 6-person party and 2-person party respectively). D&D 4th edition was based around a 4-person party. D&D has always been based around a 4 person party. Saying that BG 3 should be limited to a 4-person party because "5e" is just lacking in structural integrity as an argument.
In practice, there are many reasons why many D&D tables have consistently more than 4 players to a party. In my particular case, many of us simply don't like vancian spellcasting, or those that don't hate it are simply sick of playing a spellcaster. So, what did we do instead? We gathered a couple more like-minded players (to make up for the fact that we don't have magic to solve all our problems) and game with a party of 6. It means that we can literally play whatever we want without being expected to accommodate the needs of the party; because otherwise in a 4-person party, before character creation we would have a session that nothing but discussing who was going to play the Warrior/Mage/Priest/Scout.