I have used Vaadin and Streamlit for various small/medium projects. Apart from the obvious benefit of not having to touch JS(:D), the benefit is really, well, one codebase, one language for everything: There is no context switch, no need for various skillsets (to some extent), no need for explicitly designing/thinking about client-server communication, doing everything in the language you (or the team) know best, ...

Of course, your argument is totally valid and if things get complicated or you need to "dig deep" you have to make the separation.