Doesn't really matter.
Why?
Because the main end-game content will have a cap on how many people can be involved, which will be four (4) people. We're talking Neph Rifts, Greater Rifts, and Bosses, where most of the grinding is done, making the idea of 'more' entirely moot. More people will make doing bounties and all that annoying and even out in the Open World, your group is limited to 4, so again, why does 'amount' matter?
It doesnt. Because people who want to play Diablo 4, a vast majority of them, are coming from previous Diablo games where 'open world' wasn't a thing and will do nothing except making the completion of aforementioned bounties annoying as hell. And one thing that annoyed me more than anything in Diablo Immortal was seeing an Orange Elite and seeing it grayed out just because someone else had tagged in seconds before I could.
Your argument is a fallacy as a result of this. Diablo 4 is not a MMO, it's end game content is not meant for groups greater than 4 (looking at WoW raiding as the comparison to this).
Unless they create 'Guild v. Guild' content like Immortals has (which, frankly, not everyone wants to participate in either), having an 'MMO' experience in an ARPG is gonna end up bad, resulting in Blizz doing something similar to what Mad Head Games did with Pagan Online and turning off the MMO experience for something far more palatable to most Diablo players.