As I said over at Cameron's article, I think the reason soloing has become empowered in MMOs is not because developers think more people want to solo, but because it's easier to make both soloing and grouping viable option by beginning production with solo gameplay.
People love to group in Diablo 2, yet every class is capable of being a powerhouse on its own. What makes grouping attractive is mainly loot sharing and the ability to tailor one's skills to match any partnership. Both grouping and soloing are fun because complimentary skillsets are an option, rather than the rule. A necromancer can focus on curses, thereby playing a support role, but necromancer is not essentially a support class.
Solo-based design is more versatile.