I'd say, we make the default the one that works the best. Objectively, I'd say it's Blank, as it works for both desktops/mobiles/tablets, and we don't want a default theme to be broken. For now, this is the only theme we have that works perfectly and stick to the community spirit (minimalism, simpleness), so we should go with it.

Now if someone think it is not "cool" enough for a default, then we'll reconsider this question when a "cooler" theme will pop up. But for now, let's stick with it.