Tough questions, but fair questions.
if CZML is something that AGI/Cesium team was committed to or if it may die out
We’re committed to the format, yes. In addition to being a core Cesium committer, my primary responsibility at AGI is lead developer for our STK Components product, which, among other things, can produce CZML to visualize complex time-dynamic analysis scenarios: http://www.agi.com/resources/help/online/AGIComponents/Programmer’s%20Guide/Overview/Cesium/Overview.html
We originally created CZML specifically for this purpose, and the Components Cesium library is built on top of the open-source czml-writer, which in turn is largely code-generated from the schema definition. As AGI builds new client-server products, Cesium is our primary visualization platform, and CZML is our primary transport format for loading data into that visualization, based on server-side calculations and analysis. So will it die out? No.
Is CZML THE way to do things in Cesium? Maybe, depending on one’s need. If an app doesn’t need to visualize a time-dynamic scene generated externally from Cesium, then maybe not. The various layers of the Cesium API provide different entry points with different amounts of flexibility (primitives - update everything yourself however you want, entities - tell Cesium declaratively how it should update things based on time, CZML - write a document describing how things change based on time).
All that said, however, I have also seen that CZML has become an afterthought for the team recently, unfortunately. Personally, I will be trying to take a stronger ownership role on CZML itself in the future. I am working on adding a number of features that were never properly put into the spec, and there are a few pull requests open with some great features that I want to give a careful and thorough review so we can bring them in (arbitrary reference frames and custom properties).