Your post is great about stdlib and I just would like to put an other point to precise
on why the core team did not lot of works about the standard library.
Currently, OCaml is used by the MirageOS project. Obviously, it used in this project because we have a reliable type system, a modular system too but the weight of caml runtime is important too (when you want to make the smallest unikernel) - including the stdlib.
In this context, it’s better to have, as a third part, some useful libraries and compose them each together in one program/unikernel. Finally, we can produce a binary which correspond exactly to what you want. It seems to be a specific case for a specific project but, by this way, MirageOS provided some good and modular projects used by the community (like not be dependent on the Unix system, compile it on rasberry-pi - and others exotics architectures, use it in our web-browser, and so on).
Finally, this point is more general than the purpose of MirageOS. With OCaml, you can believe to what you expect when you compile your program to any back-end (lot of works was done on this way instead a better stdlib) and if your expecting computation diverges on what you want, you can choose an other stuff (without penalty) or, as you said, rewrite it ! To be clear, it’s better to have choice than to have all.