If the application is interesting, people will find find a way to run it on modern hardware. It's not an excuse not to open-source what you can of your code.
This line of excuses is just weak. Either release the code or keep it proprietary, that's your decision as owner. Just spare us the silly excuses and trying to make it seem like your company would really really like to open-source if only it wasn't for that tiny detail of blah...
This also becomes fun if you worked with external contractors, who committed all over the code and the agreement doesn't cover releasing the code.
And then there are still all the other things in open sourcing (making sure comments are appropriate, naming is appropriate, ...)
And then you have IP besides the source, in artwork where designers often have some rights, copyrights, trademarks ...
And after all that the "fear" on your reputation, if one fears the code might seem of low quality, which might reflect bad. (Whether it's truly bad doesn't really matter, a few blog posts or youtube videos of somebody taking worst parts can harm reputation)
It's all simpler, if you plan on doing that eventually from the start, but notable effort for any serious project after the fact.
Your other examples mostly seem to boil down to "we were not willing to commit the necessary resources to do it". Which is fine, of course.
Yes, of course. If I invest resources from the beginning for "I want to relicense and open all the code" then there is no problem. However for most companies the priority is "get this out before our competitor releases their product" and many things fall under and often neither developers nor managers have the legal knowledge (and don't care)
And btw. I fully support this on a different level: For instance for devices like (home) routers I support a scheme where a company is required to publish the software once they cancel support, so one doesn't have to throw away fine working devices only since the vendor stops fixing security issues. But that won't happen quickly, as it goes through the supply chain: You need the device drivers for the chipsets etc.
I did eventually open source some code from a popular shareware utility years ago after people asked me to. (No one ever did anything with it of course.) It took me a while because I did use some third party code but eventually decided the company that wrote the code was long gone and no one would care. But a business with lawyers would not have allowed that. And even documenting what would be needed to replace the routines would have taken a fair bit of work.
The reason to open sourcing abandonware is not because you know for sure someone needs/wants it, it's to enable someone who does need/wants it, can still have it.
Instead of assuming you know what people want, open sourcing abandonware would enable people to make that decision for themselves, not you making it for them.
There are examples of niche things being resurrected by players. Niche MMOs have from scratch servers, niche consoles have complete emulators, etc. Saying "no one is going to go to the effort" is not accurate.