The Open University is using a theme based on bootstrapbase, and that was only 'finished' a year ago. Of course, we are still working to improve it. Doing a custom theme for the whole of Moodle is a huge job, we want to use this theme for several more years, not throw it away and start again.
Base themes in Moodle core need to be built to last a very long time. Really, we should be able to update third-party dependencies in-place, rather than having to continually throw them away and start again.
I suppose we have not thought about how hard it would be to change the theme we are based on. However, since themes have to have a lot of code to override core stying, I am sure it would not be pretty.