Welcome to the Cesium Community!
Thank you for keeping up with the changes in CesiumJS. We have had a similar discussion about the development of CesiumJS which you can view and contribute to here: Master branch is not receiving much commits anymore?
Like Lisa mentioned in the discussion, CesiumJS is at the heart of our platform. The recent slow down in CesiumJS development has been a result of focus on exciting products that will contribute to the whole Cesium platform. That being said, this is not a long-term shift and we plan to continue maintaining and updating CesiumJS to serve the needs of our community.
We would love to hear about you and how you are using the platform!