Fair. As you said, I think the 4 party limit still allows a fair amount of variety in that regard. Those different roles can be filled by quite a varied amount of classes, for exemple the wizard "archetype" can be filled by both the wizard, the sorcerer, the warlock or even the druid with the right build. At the same time, you can play the same druid as a defensive fighter if you want to. Builds can be flexible enough to not having to be limited to one cookie cutter composition every time.
Sure, I agree with all of that.
Imo, a bigger problem is that a 4-player party restricts you to 3 companions. Given that BG3 is expected to be a 80+ (100+? 120+?) hour game, many people probably won't want, or don't have the time, to replay it. These players will miss out on a ton of content - party banter, companion quests, companion character development, etc. Assuming 10 total companions, and that some players will refuse to play with ~2-3 companions because of personal dislike, a party of 6 means that these players can experience almost all the companions they'd want to in a single playthrough. Edit: this is an especially important argument if Larian kills off non-party companions after Act 1, rather then allowing them to chill in camp and be swapped throughout the campaign.
Also, there's the whole argument that larger parties allow for a wider range of combat tactics and character synergies, enabling cool combos.