I think that witnesses are a vital part of this place and therefore the work they do should be reflected in their score.
As far as the power that a small group of individuals have to essentially lock people out, can get a bit tricky. I like the idea of a smaller number of votes, which would mean less control of the top 20.
I do unfortunately think this would hurt witnesses like, @Curie the most though. Seeing as the witness is solely for the platform, non profit and doesn’t partake in the politics associated sometimes with the vote for vote activities on the platform. Many large accounts may choose to select witnesses that give something in return. Even though, I think the curation of valuable content on this platform is beneficial to everyone and therefore should be supported more than it is now.
I like the idea, and I’m curious to see where you go with it.
Also, something to think about- There are other individuals who do a lot off platform, whose contributions are not currently reflected in the ua score. Developers, curators, etc.. it might be worth trying to add them in somehow.