There are multiple unstated premises in this question. Presumably these people are at least users of the software. Also, they'd need to have something they want changed. Further, those changes would need to be changes they want to contribute. For example, changes that tailor the software to my environment or workflow are not of interest to others.

There also seems to be an implicit and, in my opinion, obviously false assumption that a substantial fraction of users of a piece of open-source software should be contributors to it (even restricting to those with the technical skills to do so). Ideally, most of your users are happy with your software as-is. Of the ones that do want something to change, their options are: live with it, switch to different software, contribute a change, rewrite the software. While it depends on the software and desired change, in most cases there are orders of magnitude differences in effort between each of these options.

Presumably these people are at least users of the software.

I don't think you can presume that. When we talk about contributing to FOSS, we think of someone who wants to write code and solve problems in someone else's program. Whether they themselves use the program is tangential. If you do a strict TDD, they could get away without running the program at all.

users of a piece of open-source software should be contributors

No, I'm asking about people interested in programming for FOSS, not users.

I think it's quite safe to neglect. Virtually no project is going to have a significant fraction of its contributions coming from non-users who are only motivated by a desire to contribute to FOSS. Such people will have plenty of FOSS projects that they use that they could contribute to and/or are motivated by some mixture of gaining reputation, impact, a chance to explore something they're interested in, or learning. The vast majority of available open-source projects such a person could contribute to offer relatively little for all of these. Even selecting projects uniformly at random, this would lead to a tiny fraction of contributions in a project coming from such people, but the actual distribution is likely more like a power law. The vast majority of projects receive essentially no attention from such people, while a few projects receive a lot, e.g. the Linux kernel.