I don't think this is really advocacy for monorepo but rather advocacy against premature solidification of abstractions. Whenever I encounter a situation like this, the first step I take is to write the code without any API or interface boundaries to worry about. Then if I still feel it would be a good idea to separate it, I will separate it while keeping it in the source tree (this isn't really vendoring or monorepo, it's just abstraction into a module). Once this thing has survived a few rounds of revisions and you're ready to use it in another project, copy paste it first. Then once two projects have both had a chance to exercise the abstraction, then you factor it out into something totally separate

This process requires that you are able to work on rewrites and refactorings without having to beg. But it's truly the best way to get from an idea of a library to an actually well designed library.

In some situations it might be you already know what you want and can separate it out from the beginning, but if you are coming up with anything remotely novel, then you want to follow this process.

A monorepo will still contain libraries like these. It will also contain vendored code (although monorepos help reduce the amount of duplicate vendored bits of code). Making such changes will be easier in a monorepo once your abstraction starts to solidify. But the process you should go through should still be similar. The reason for not having two things depend on your wobbly abstraction too early is simply that it will inevitability lead to you prematurely solidifying uncertain abstractions.