While it is true that things don't move fast, there have been proposals to it which have seen some adoption (thinking MONO_BUFFER was implemented by smlnj has compat in mlton). And I recall a proposal from polyml subsequently was implemented in smlnj. I don't know if there actually is a process in place to actually decide to accept proposals into the basis.

One of the things I like about the basis library is it's principled stance that if it doesn't require compiler support, host operating system support, is more efficiently implementable by the compiler than would be with an external library, or has clear proven utility it belongs in a separate library. This barrier for inclusion is to me at least undoubtedly good.

Personally I would split things apart further (kicking host OS support to a separate library from the basis, as I am inclined towards non-posix hosts where most of the API requiring host support isn't implementable). And kicking out any optional types/modules to separate libraries (where said library is then optional).

So while it is about as minimal a standard library as exists, (for my purposes at least) it sometimes feels like it contains too much. Given there is no language level support for things like conditional compilation of anything optional within the basis. These tend to be my gripes with the basis... not "it needs more things".