Something that's interested me is how hard drive capacities have continuously gone up, but read and write speeds have not really changed. To the point where it would take multiple days to read these new high capacity drives.

I wonder what the use case for them is, they aren't ideal for RAID as they would take far too long to rebuild the array in a drive failure, but they also aren't great for long term cold storage. Perhaps they are ideal for periodic backups where you want to take a daily 1TB backup of a server or CCTV where you don't really care about backing up the data as it's all deleted on schedule anyway.

I imagine that there is a ton of data in the "would be nice to retain but not so critical as to warrant a constant backup" category. Alternatively stuff that requires large storage, but only occasional small writes, so incremental backups are feasible.

Most of the data on my NAS is of that form.

You use them for backups, data archiving, or object storage with triple parity RAID or erasure coding. The combined bandwidth of dozens to hundreds of magnetic hard drives is pretty decent.