The problem I see is that it becomes a cycle when the leves difficulty is adjustable because most people will choose the easiest route (ie: solo) because of whatever reasons, then eventually less people want to team up, more choose to solo...etc etc. Leves are anti-social, and they're killing the community.

They should separate leves vs regular leveling up with mobs. Make one easy and doable for solo and the other one challenging but requires teamwork and more rewarding for those who want efficiency.