The developer time isn't higher than if it was server side rendered. And for some applications, like the ones I'm building, it would be impossible to do the work on the server.
I wouldn't say that it increases developer time. From my (sure, limited) experience of building both with Flask (Django-like, Python BE framework) and Svelte, the difference between developer time is negligable, if not in favor of building on the client - the component frameworks bring much more to the table, like scoped CSS, often better editor support for dynamic values in the HTML templates etc.
