Any quest should be soloable on any difficulty. But that does not meant that it should be easy so anyone can do it. That is how I test how powerful my toons are. That is how I learn different tactics and strategies. I do that to challenge myself. If I beat the quest, then I try to beat it faster or with less resources. But playing MMO always solo? That is not why I'm here.
edit: btw my greatest accomplishment so far is The Pit in 13min.