Honestly the only valid arguments I see against an optional 6-person party are:
1.) Working on this will take away Larian's resources from something else that the poster thinks is more important
2.) Poster will feel compelled to have the full 6-person party so they miss less content, but think this will be less fun in a TB game especially with Larian's UI. And self-imposing oneself to 4 characters is not fun, for the same reasons that "If you don't like it, don't use it" is a bad argument: imposing self restrictions is not fun or good game design.
3.) Larian will implement a 6-person party in a terrible way that affects all players, not just 6-person-party players (e.g., adding more enemies, making the 6-person party required, and/or using the 6-person exp mechanic for solo players)

These are the only things we should be discussing. Most other arguments boil down to essentially: "I don't want to play with a party of 4, so no one should be able to without mods."

The responses to these concerns are:
1.) It can take negligible work to implement a 6-person party. Larian could simply make it an option without changing anything else. Or they could add [optional] scaling exp, which is very simple.
2.) The game shouldn't allow a 6-person party by default. You should have to go into settings to enable it.
3.) Yes, they might. But we're arguing for what we want, not what we think is going to happen, hopefully so that Larian takes inspiration from our ideas.

Similarly (to be fair), people arguing FOR a party size of 6 shouldn't be using arguments such as "a 6 person party matches BG1&2" because similarities don't necessarily equal better. 5e != AD&D 2e.

If you have a different argument than the 4 listed above, feel free to mention it. I'll admit it's likely I'm missing something.