In a perfect world, I don't have an issue with 6 player parties. They could have the game the game perfectly balanced for a lone wolf style playthrough, 4 players or 6 players.

In practice, that balance is hard.
In BG1, the XP cap was easily reached even with a 6 man party, so running with anything less was shooting yourself in the foot.
in BG2, as a party of 4 (not so much with 5), you would get HLAs much earlier and you would get much more. This made a 4 man party considerably more powerful
in DOS2, lone wolf is considered to be a lot more powerful than 4 man party.

There is also another knock on effect. If you allow 6 man parties there need to be more companions available. Would you rather have more companions which are more shallow or fewer companions with more backstory and quests? I suppose this is down to personal preference, but I would choose the latter. Replaying BG2, I hate how shallow most NPCs are. Minsc, while much beloved, does not even have a side quest. He only has a couple of funny lines in dialogue now and then.

You also need to consider that the devs don't have infinite time to work on the game. Would you rather have them spend that time balancing the game for different player counts? Or would you rather have them spend that time on more side quests or adding more replayability?

I also do not agree with all the reasons for introducing higher player counts:
  • Higher player counts is not a solution for the difficulty of combat - that needs to be tweaked by itself
  • Atomic party is not that needed anymore - anyone can do rogue skills, almost all classes have access to (utility) spells. The most needed role is probably a healer with how much dmg you take in combat, but in theory, short rests were added in 5e to reduce the need for healing

In the end, while I am not opposed to 6 player parties, but I don't think it adds much value either. I guess we will have to agree to disagree...