Language design and compiler implementation are definetely high-competency skills, but they don't necessarily correlate with library design skills.

For instance, http client and server libraries are often in this gray area of uncertainty about whether they should be in the stdlib or not. Is this something language designers or compiler implementors have a lot of experience in? I would say not; sending and serving http requests are not something compilers need to do. Or take GUI libraries, what do language designers know about that?

I also know of many bad third party libraries, but there are tons of examples of really awful parts of standard libraries. The original date/time APIs in Java are a mess (they finally fixed this by in essence bringing in a third party API). The ssl bindings in the Ruby stdlib were a common source of bugs back when I was paying attention to this (maybe they've fixed it), same thing for the built in http stuff. Someone else mentioned the similar weakness of python's built in http, such that most people use a third party library instead. Even the java collections APIs are pretty poor such that people often augment them with things like guava or apache libraries.

My point is just that developing good libraries is a hard thing and I don't see any reason to think language designers or compiler maintainers are any better (or worse!) at it than other people. There isn't really a shortcut, you can't just cede authority to the powers that be on the core language teams, you just have to evaluate the quality of libraries for your use case yourself.

You've made a very good point that proficiency in developing compiler infrastructure does not imply that you're qualified to develop every specific aspect of a standard library. Date/Time, as you pointed out, is a very good example of this. It's a very complex domain that requires specialised knowledge. I'll counter this by saying that one aspect of language design is choosing the scope of the project, and deciding how best to implement a standard library targeting the language's intended domain. If your language is designed to implement web servers, then developing a GUI library might be a poor investment. Consequently, if your language is designed for implementing system applications, then investing time and talent into developing things like FFI, Filesystem and GUI functionality are just the prerequisites to the language being useful in its intended domain.