Hey, Matej here, I'm part of the UX team at GitLab. I understand that it can look like we don't spend much time polishing the user interfaces and experiences that much but I can assure that we do. We don't have a split in % on how much time we should spend polishing existing things but maybe it's something we should consider doing.
Just to give an example of a recent redesign of an existing feature that I was involved in, it can be found here: https://gitlab.com/gitlab-org/gitlab-ee/issues/6089
Our application covers the whole DevOps lifecycle which means it takes a lot of effort to upkeep existing and constantly ship new features, but we always strive to do our best.
Thanks for the feedback, I think it will trigger internal discussions on how much time we should spend polishing existing features and experiences.