If a library is supported and maintained by the language designers, then its lifetime will be at least as long as the lifetime of the programming language. Or if the library is replaced, then it will be done through some mechanism of deprecation, rather than sudden abandonment.

I don't see the standard library and third-party packages as being in opposition. Far from it. A rich standard library encourages a rich ecosystem of third-party packages.

Maintenance costs affect the authors of open source packages even more than they affect commercial users. They are working for free and in their spare time. A rich standard library makes a language attractive to people who want to write packages.