I think they should allow a single player to start their own guild as well. As we all know, people have been getting around this requirement for a long time by recruiting temporary groups anyway. If they introduce a method to allow one person to invite their own alt, I don't see it as being a big deal to remove the arbitrary restriction of needing six people to start it up in the first place.
Five years ago there would have been a firestorm of "no way"s on this idea because it "wouldn't promote a sense of community" but these days I don't foresee many people objecting.