If anything, it seems you are the one attempting to justify your own arguments, and you're making a number of assumptions to do so. You're assuming, for example, that BG3 will be exactly like DOS2, that party members will die at some point - among other items you mentioned. Though it is possible, and maybe probable, it is still an assumption. They certainly don't have to do it that way, and in my opinion they shouldn't if they don't want even more people griping about how this is a DOS3 and not BG3. You are also assuming that if they increase max party size to 6 that they have to then increase multiplayer max party size to 6. They COULD do a max party of 4 players with a max party of 6 or even 8 (2 to 4 NPC members). In fact, if they really wanted to make some people happy, they'd make it so you could play a party of 4 players with each player being able to control an origin character and allow up to 8 max party size with 2 characters per player
Anyway, my point is that there is legit reasons why some of us want a party of 6. It's not just, "I want a party of 6, so you people who want party of 4 just shut up." Again, party of 4 limits the game and locks out content and makes things less fun, especially since I've played the mod and enjoyed party of 6 so much more. It's like getting a taste of something good and having people tell me I can't continue to have it. Screw you because I only want my limited version.
But I'm asking for both, and that's why I said they COULD balance a party of 6 by making a 5e Core ruleset and allowing 4 custom characters to start, even in single-player mode. Then those who want to play party of 4 can do so on the janky homebrew ruleset and those who want to play hard core 5e AND party of 6 can do so. They wouldn't even have to add or remove more monsters. Then everybody is happy.