Backups are one thing, but long term archival is another. For example, you might be required to store emails for 7 years, or keep all project data indefinitely. I used to save archives to tape, but then I've had tapes get destroyed (drives rip the tape out). So...write to 2 tapes I hear you say. Is that what others do? Have 2 (or more) tapes of the same data for redundancy?
But then the other issue is that tapes cannot usually be read by different backup software vendors. Eg if you go from Arcserve -> Backup Exec -> Commvault over 10 years you would need to keep all 3 systems so that you could restore old data. Likewise for hardware. Old tapes might not be barcoded. Might not be compatible with the new library etc etc. So do you keep old tape hardware AND old software just in case you might need to restore a 10 year-old file?
Or...when you move to a new backup system do you migrate all archived data to the new system and re-archive it onto new tapes? That could be a huge job.