Java applets were never ubiquitous the same was JS is on the web though - there's literally a JS environment always available on every page unless the user explicitly disables it, which very few people do.
JS is here to stay as the main scripting language for the web which means there probably will be a place for node as a back end scripting language. A lot of back ends are relatively simple CRUD API stuff where using node is completely feasible and there are real benefits to being able to share type definitions etc across front end and back end
> there are real benefits to being able to share type definitions etc across front end and back end
There are benefits, but cons as well. As you point out, if the backend is only straight proxying the DB, any language will do so you might as well use the same as the frontend.
I think very few companies running for a few years still have backends that simple. At some point you'll want to hide or abstract things from the frontend. Your backend will do more and more processing, more validation, it will handle more and more domain specific logic (tax/money, auditing, scheduling etc). It becomes more and more of a beast on its own and you won't stay stuck with a language which's only real benefit is partially sharing types with the frontend.