Server-side JS is an exceedingly useful tool for certain use cases. Specifically, sharing server and client logic. I know everyone here likes to hate on JS frameworks, but the productivity of being able to build one thing in one language instead of the same thing twice in two languages is hard to overstate.
Sure, that has some trade offs like page bloat. But almost all of the JS framework universe is focused now on reducing or eliminating those trade offs: partial hydration, build time code elimination, server components, etc. There’s still a long way to go before the DX and UX reach parity, but it’s inevitable, and closer than one might think (see Qwik[1]).
There's no reason to use two languages. All the major server-side frameworks still exist and many have been upgraded with the modern component style templating for the frontend too.
Nothing is new with JS frameworks. Hydration and fancy "resumable" state has been done over a decade ago with serialized viewstate and AJAX controls. The JS world is just discovering how powerful server-side stuff can be after all this time.
Sure, that has some trade offs like page bloat. But almost all of the JS framework universe is focused now on reducing or eliminating those trade offs: partial hydration, build time code elimination, server components, etc. There’s still a long way to go before the DX and UX reach parity, but it’s inevitable, and closer than one might think (see Qwik[1]).
1: https://github.com/BuilderIO/qwik