Everything written on a DC-600 class cartridge is readable on the 4GB model I have around; I didn't treat the DC-3000 class tapes as long-term archival, nor the 8mm's; the last DAT will read everything older; the SuperDLT 220 reads all the way back to DLT I; an LTO-5 reads all the way back to at *least* LTO-1, and possibly the SuperDLT formats.
In short: yes: tape drive hardware design engineers are nearly fetishistic about backwards compatibility, drive longevity, and interchange.
And you're correct in saying that it's generally not a *requirement* to read data that old -- but the fact that you can usually contributes to making it easier to read data that's much newer.
One other reason, BTW, that that was possible?
Non-proprietary *backup software*; those tapes were written with a "super-Tar" program, Microlite's BackupEdge, I think, so that even if that
software was either no longer available or incompatible, I wouldn't care: *tar* could read the tapes.
You're certainly correct, though, in noting that for "true" archival storage, it's a job in itself, migrating to newer formats and layouts. This would be a vote in Obnam's favor; at least you could keep the package around in source, and as long as you could make it build, you could retrieve stuff.
I have in fact had to retrieve 5 year old backups, of accounting year-ends when a merger was being contemplated. That was when my client turned out to be really happy about all that money they'd had to "waste" on backup tape (nightly full, 5 Friday's, 3 EOM, 4 EOQ, pre-close and post-close).