We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time (tm) ). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
Easiest way to add a ton more storage is an HBA card with external SAS ports, and a JBOD disk shelf. A disk shelf is basically just a power supply and a SAS expander card hooked up to a bunch of disk backplanes.
You can find them used for reasonably cheap, or build your own from old parts if you're on a shoe string budget and don't care about hotswap bays or anything.
They will act just like any internally connected hard drive, so you can make them part of whatever RAID or ZFS arrays you want.