You can't compare the PvE times of a person to a party of 5, because the party will have specialized roles and no downtime. As a solo person, you have to sacrifice damage, sustain, or tankiness, which means that you'll either kill slower or have downtime.
Going back to the topic, the game should have solo content, but the focus should remain on coop. If we don't have much solo content now, it doesn't mean we never will.