A number of languages are trying to be relevant in different places. Kotlin, Dart and JS (as examples) focuses on backend and frontend development. Kotlin and Dart also offers mobile options and JS I believe offers it via some third-party libraries.
Depending on your toolset, anyone can be a frontend and backend developer, you might not be great with CSS, but you will still qualify as a full stack developer since your knowledge of Kotlin will allow you to write KotlinJS which will allow you to build functionality in the frontend. A person who is great with frontend development using Dart might not be great building backends or writing database queries, but they will still be able to contribute to a Dart backend.
Unless your frontend / backends are overengineered with a gazillion frameworks each specializing in a small part of the system, I see no reason why a frontend developer can't work in the backend as well (assuming they can use the same tools)
Full Stack Developers will not go away, they are especially useful in smaller companies where the budget doesn't allow hiring developers with specialized skills. Larger companies, specialized developers will probably be the norm in the future.