> for library writers, how many people might use it can be a deciding factor on investing the time
Yes. Especially given that a high percentage of library development these days is being done by industry, not just academia. (At least in machine learning.) They're not writing packages for fun or for academic citations, they want users.
Julia does appear to have a higher than usual proportion of package authors to total users, FWIW. I think it's both a blessing and a curse, because feedback around gaps in the ecosystem is sometimes brushed away with "write your own package". That is, because a large part of the community writes packages to scratch their own itch (I've yet to see one written for citations), they expect others to do the same (which may not be fair).
Yes. Especially given that a high percentage of library development these days is being done by industry, not just academia. (At least in machine learning.) They're not writing packages for fun or for academic citations, they want users.