IMO, one of the core reasons why UI/UX in open source applications (that aren't backed or actively contributed by dedicated staff in big companies that keeps them on a salary for the sole purpose of contributing) is because it's more widely accepted for programmers to contribute to open source than UI/UX designers.
In this case the best one can do is design mockups, but then who implements it? Who even has to be convinced that the mockup is a step forward or backwards, the moment every design decision is treated as having the same value as the next one, on the grounds of "it's just an opinion"?
My point is, we don't lack programmers/developers we lack UI/UX people willing to contribute, and we also lack the former to actually listen to what they have these people to say instead of dismissing their suggestions as "not data driven enough", "too opinionated" etc.