A studio should be able to afford a good LTO tape drive for at least one backup copy; LTO tapes last over 30 years and suffer less from random bitrot than spinning disks. Just pay someone to spend a month duplicating the entire archive every couple of decades. And every decade you can also consolidate a bunch of tapes since the capacity has kept increasing; 18TB tapes are now available: $/MB it's always far cheaper to use tape.

They could have done that with the drives, but today you'd have to go find an ATA IDE or old SCSI card (of you're lucky) that'll work on a modern motherboard.

But I'd guess their problem is more not having a process for maintaining the archives than the technology. Duplicating and consolidating hard drives once a year would have been relatively cheap, and as long as they verified checksums and kept duplicates, HDs would have been fine too.