"ZFS isn't even a filesystem for this age" - WTF does that even mean?
It means that even back when FAT was a johnny come lately it already had greater market penetration than ZFS. With decades behind it and broad market penetration today, there's good reason to believe it won't vanish with the advent of the next development in filesystem architecture. ZFS is likely to be a blip on the radar, a pause before the next innovation. Not what you want for an archival format.
Bit-rot is an issue inherent to any storage medium
Bit rot, aka corrupted data, is not inherent to correctly operating hardware. As implemented, you'll see tens of thousands of unreadable blocks on a hard disk before you see a single one in which data has been undetectably corrupted. Every single sector gets a checksum in hardware and if the checksum does not pass you get the famous Abort Retry Ignore. For most storage you get Forward Error Correction coding so that some number of bit errors can be corrected on read before having to throw an error.
When you see bit rot, the storage media is usually not at fault. More often the data passes through faulty non-parity ram, a noisy memory bus or an overheated controller and gets corrupted on its way to storage rather than getting corrupted at rest on the storage. It died when you used an overclocked piece of garbage to copy it from an old hard disk to a newer, bigger one.