D4 will already do that. There are many that will not be doing the MMO aspect of the game. There are dozens of Clans that have already banned taking part in the world bosses. Then you have those that will only ever play alone or with their small team and never with random people.
Being online doesnt protect from anything. I know of at least 100 people all using the same set of 10 accounts. A VPN and shared log in details defeats that easy enough. So that argument is dead in the water.
We will have to see what happens when it comes out in 3 or so years.