See wazoox's comment: per the spec/in theory and in practice for him, you can read back 2 generations of LTO tape (and write back one, to ease transitions).

Any sane archiving plan will read old tapes "before it's too late" (yes, I know, I know) and re-archive them. I suspect a prime use case for this proposed optical media is archiving what Hollywood et. al. does, which is getting to be rather large (10s or 100 TB per a major film?), put them in controlled environment facilities and then forget about them unless the film is such a success you want to make a new release that needs to go back to the masters, or more material for a "Director's Cut" etc.