Those are completely unrelated things.
First, many people aren't aware that some of the plugins are open-source. Before it was brought to my attention a year ago, I didn't even know there were open-source plugins, because when you look for the source code of MusicBee you don't automatically find the source code of the third-party plugins. Also, when you type "UPnP/DLNA MusicBee" on Google the repository does not even appear on the first page of results on Google. You have to actively look into some of the pages to find it.
Second, you can be interested by helping to improve some parts specifically. If you're a web developer for instance that does not automatically make you happy to work on *anything* web-related. The same applies here. You can be interested to dig into the audio processing source code, into the UI kit, into the panels management, into the plugins system itself, etc. - not in the UPnP/DLNA part or what other plugins are trying to achieve.
Third, maintaining an out-of-date plugin that hasn't been maintained in *years* is completely different than working on MusicBee itself, which is regularly updated.
Fourth, some people people may want to look at the source code to understand how the program was made and how some of the tricky parts (e.g. sending processed audio directly to drivers through WASI & others) were done, how they where optimized, and so on.
Fifth, making the program open-source may multiply the number of potential developers who could get interested in the project, as this has been the case for a lot of large projects in the past. You can take a look at VLC, or MPC-HC for instance.
I know this has been discussed again and again, but just saying that because no one maintained a few plugins that only a very few people are even aware of the availability of their source code, is plain stupid - and I'm sorry if this seems offensive, but I'm also tired of giving the same arguments again and again while you seemingly don't understand them.
I can fully understand that Steve doesn't want to make MusicBee open-source, as they are plenty of reasons for that (you don't want to take time looking at PRs, you don't want someone else forking your program and making its own version, etc.), but I'd still like that you aknowledge the many (MANY) reasons there are to want to bring a project to the open-source community aren't something plainly stupid or unreasonable, even with the way things are currently.
And as a quick reminder, I'll say again that making a project open-source doesn't mean accepting contributions. You can just expose the source code - there are even paid software that do that, specifically because it helps other developers to understand how things were made and get inspired by them.
Nonetheless, I with you a good day and a good evening and hope you'll understand the things that bother me in the current situation.