It's probably worth noting that Plasma's customization support is useful not only to people looking for a special look & feel, but also as a proving ground for new things (and new approaches to old things) that can eventually make their way into mainline Plasma. Making things harder to customize would mean fewer people experimenting, refining ideas, and solving problems. With that in mind, I think that removing customization in hopes of simplifying test cases would be a mistake.

However, it's my understanding that some of Plasma's customization hooks allow third-party theme components to run arbitrary code, without even a warning, sandbox, or reasonable way for the user to inspect it beforehand. (This was in the news a couple months back, IIRC.) That was more or less okay 20-25 years ago, when malware on Free operating systems was almost unheard of, but it's dangerous and irresponsible in today's world, where extensions/plugins have become common attack vectors. I wouldn't mind a little loss of customization to shut down that vulnerability, at least until safer extension APIs can be built.