Something else: I support some long term clients still using some 1990s software that I've ported to run within a VM on modern hardware. I ended up using developer tools made in the 2000s to decompile the executable and make changes to the software, others have done so and posted their own version. These tools were not available when the developer walked away, so they probably never envisioned these changes.

With AI and other tools developing rapidly, I wouldn't be surprised if someone was able to do things with your code in ways you didn't expect, even if you choose not to open source it yourself.