In Yoshi's latest statement, he said he wanted the early stuff to be soloable. And it appears that as the story develops you *need* to interact with people (as after you start the dungeons there is less solo more party play. I doubt it will ever go to 100% party only. But there is definitely more party play.
People need to wait until we see stuff past the start. The start should be something you can solo, and since we are really only seeing up to ~20, with the ability to level beyond that really throws off perception a bit.
Be patient, it is a slow process to move a mountain.