In my experience, what starts off as a simple viewer feature creeps until it is a complete editor... however, I would quite like something similar to OpenSCAD that works with WRL, STEP models.
There are several parts of KiCad that would make useful standalone tools, I don't think the devs imagined those use cases. The trend for KiCad is closer integration. Indeed, previously people used the tools separately, and they have lost some of that ability. (e.g. just run eeschema as a drawing tool, or just pcbnew for layout).
I think it would be quite some work to split out parts, and maintaining them would be "swimming against the tide".