I don't even think this necessarily has to do with open sourcing necessarily. This is a wider software conundrum as well. From the article:
> The first warning Shepard got was when she went to update her Mac, and the system warned her that TropeTrainer wouldn’t run on the newest OS
Operating System updates frequently break software. Operating System transitions break software a lot. This is a thorny issue, imo, because on the one hand when people buy software I doubt they're thinking about how it'll be useless if the Operating System's company (or community) think they can do better with new APIs and behavior. On the other hand, updates are part of moving something as vast as an OS forward, and at least some amount of the time they're necessary.
Windows provides some ways of running older software in a VM. I'd be curious if they've tried such a thing with this software. MacOS I'm not so sure of, since even the ISOs are somewhat controlled.