I kinda stopped after the first point because I just want to state the counter-argument to your thesis:

The idea of 'fun' differs from player to player, as different factors are set for 'fun' for players. What you believe is fun for you isn't necessarily the same for someone else. I'm quite comfy with my group of players I play with which are no more than 8 players, and I don't need to be in a group of 100 people to down a boss, so long as I feel like I'm having fun with the players I am with.

So in the end, your entire point is more of an opinion base than an argument and cannot be held up solely because it's based on a bias.

Sorry, I rather just not go into a "my opinion is better than yours" debate. They tend to turn into bitchfests.