I’m sure you don’t agree with me. I’ve read many of your posts before 
It’s a closed list for two reasons–
First–this forces a normalization to a clean, consistent set of names for credit roles, so there aren’t seven ways of saying the same thing. I’ve seen an early version of Roon (never released to the public) that accepted and displayed credits indiscriminately. The experience was unclean, cluttered, and repetitive, and the only way to fix that was tons of manual editing–something that the great majority of people aren’t willing to do.
Yes, there are some repeats in the list we are using–they didn’t originate here, but they do exist. We should clean those up, and when we do, those benefits will filter to everyone, not just a minority who are willing to edit.
Second–If we know the full set of roles at design time, there is more power to do interesting things them. This has been important recently…we have been working on machine learning stuff that needs to know if a credit role is an instrument, a producer role, an engineering role, a composer/conductor/etc role, if it is a keyboard instrument or a type of trumpet, etc. By working with a fixed list, we can manage these classifications centrally.
Genres being an open set is problematic, but the horse is already out of the barn. For example, Roon Radio doesn’t know about your personal genres, nor will any of our other “smart” stuff in the future. If you do turn on user-provided genres, there is almost always mess to clean up, and manual hierarchy-building to be done–which few people do, so it gently harms their experience with clutter.
The difference between genres and credits is that there are a lot more people who’ve invested time into personal genre classification systems. We first released Roon with a closed set of genres, and that quickly became a significant barrier to acquiring users, so we were forced to make a compromise. There is not a similar amount of concern for credit roles in the world.
