This should be illegal, companies should be forced to open-source games (or at least provide the code to people who bought it) if they decide to discontinue it, so people can preserve it on their own.
Computers might be able to account for every bit with the use of parity files and backups with frequent parity checks
Yes, and this can be done through mostly automatic or distributed processes.
even the most complex system of data storage can fail or degrade eventually.
I wouldn't describe it as complex, just the bare minimum of what is required to actually preserve data with no loss. All physical mediums may degrade through physical processes, but redundant systems can do better.
but the fact is most people aren’t running a server with 4 separately powered and monitored drives as their home computer
It isn't hard to seed a torrent. If a group of people want to preserve a file, they can do it this way, perfectly, forever, so long as there remain people willing to devote space and bandwidth.
We live in a world of problems, like the YouTube problem, compression problems, encoding problems, etc. We do because we chose efficiency and ease of use over permanency.
All of these problems boil down to intent. Do people intend to preserve a file, do they not care, do they actively favor degradation? In the case of the OP game, it seems that the latter must be the case. Same with Youtube, same with all those media companies removing shows and movies entirely from all public availability, same with a lot of companies. If someone wants to preserve something, they choose the correct algorithms, simple as that. There isn't necessarily much of a tradeoff for efficiency and ease of use in doing so, disk space is cheap, bandwidth is cheap, the technology is mature and not complicated to use. Long term physical storage can be a part of that, but it isn't a replacement for intent or process.