Because if all JS were to go away tomorrow, and all CPU processing happened server-side, what would that change in server resource requirements?
> When you push the CPU processing of the UX from the mobile device back down to the server, the energy consumption certainly moves. Does it decrease, though?
One could argue that developers/website owners might be more motivated to optimize their server code for performance, thereby decreasing their hosting bills, vs optimizing front-end code which they don't pay to run.
We all can over approximate in our preferred direction.
That simply isn't true. Your device does load a library (usually minified), and parses the JS to be sure it is runnable. But it doesn't run every function just because it is loaded in memory. Even if it did, it happens on app load, once... then runs functions on-demand. And the size of the load can be mitigated with appropriate use of tree-shakable libraries. But without JS, the server regenerates an entire page, every time you do something, which takes some resources. It sends it over the wires, which takes some small amount of resources. The mobile device loads an entire page again.
I do agree there is wasted processing in both scenarios. But that waste doesn't just disappear when you do server-side work.
Their machine is already on and idle, by pushing processing onto the backend your just creating a provisioning issue where either under or over provisioning is a waste of people's time or resource.