This is only realistic for established applications with large userbases. For new or very niche apps, distros are understandably not going to be very interested in doing this work. In that case the developer needs to find a way to distribute the app that they can reasonably maintain directly, and that's where containers or statically-linked binaries are really convenient.
I agree with everything you said up to this. We're talking about a software library, for which the user is a software developer. IMO a software developer should be able to package a library for their own distro (then they can share that package with their community and become this package's maintainer).
As the developer of an open source library, I don't think that you should distribute it for systems that you don't use; someone else who uses it should maintain the package. It doesn't have to be a "distro maintainer". Anyone can maintain a single package. I am not on a very mainstream distro, and I still haven't found a single package that I use and is not already maintained by someone in the community (though I wish I did, I would like to maintain a package). My point is that it really works well :-).
I disagree with the idea that we should build a lot of tooling to "lower the bar" such that devs who don't know how to handle a library don't have to learn how to do it. They should learn, it's their job.
For proprietary software, it's admittedly a bit harder (I guess? I don't have much experience there).
This isn't really true, Fedora, Debian and Arch have huge numbers of packages, many very niche. You might well need to make the distro aware that the new program exists, but there are established routes for doing that.
Arch particularly has the user repository where anyone can submit a package and vote on the ones they use most often to be adopted into the community repository, yes.
It’s a great way to start contributing to the distribution at large while scratching an itch and providing a service to individual projects.
This is not grounded in reality. Look at popcon or something like it. It is a nearly perfect "long tail" distribution. Most software is niche, and it's packaged anyway. It's helped by the fact that the vast majority of software follows a model where it is really easy to build. There are a lot more decisions to take with something like Chromium, which perhaps ironically is also the type of software which tends to package its own dependencies.
