I don’t know about Rust or Ruby but I feel that over the last year the team at CDCK did an amazing job with the frontend rewrite to Ember 5 and Glimmer components. It actually went together with a re-build of the frontend with standardized components and styles. I’m impressed about the coordinated effort and purpose that was put behind this 
So to me they made a great decision about what to modernize because it made a huge difference in keeping Discourse modern and a joy to use!