It would be easy to build a bike with just seven speeds. Just a single ring up front. Let each rider pick the exact seven speeds he wants. It would also be easy to design cluster that snap on and off, as easy as changing a lightbulb. So, snap off the seven speeds you use for commuting to work, and snap on the cluster that you need for loaded touring in the mountains.

But, from a marketing point of view, ten speeds sounded better than seven, and thirty speeds sounds better than ten.

The ironic thing is, "Joe Average" mostly rides between 40 inches and 90 inches, yet some of those "30 speed" bikes do NOT provide seven distinctly different speeds in that range. A well designed seven speed bike could easily.