While I'd hope professional backup tape is a little better than high end early 1990s VHS tapes, the 10-20 years that beagle3 expects is also in line with what is generally expected out of modern backup tape. Granted, we are talking about archival storage, and it is still used for that, but after disks got big enough that tapes were seldom used for more than backups the pattern is to continually refresh them with multiple backups from disk.