By the time a web app has megabytes of JS and fetches just to show its initial content, is it actually saving money?
If most of that is cached in the browser, then theoretically, yes.
But then the average smartphone can't handle the JS and works so slow that the money you saved from not having a backend is never going to be recouped by your high bounce rate (i.e. people leaving the website out of frustration of its slowness).