the implications of the deleted tapes might not be know for years to come but they will be needed eventually - or rather they could have been used but now that option has been removed.
It the subjects of the communications were important enough to tape in the first place they must have had some doubts about them. Most terrorists do not wake up wanting to bomb buildings, they are radicalised and taught, maybe by the people on the tapes who knows.
If nothing else having them on a block chain means they would never have been lost and with the amount of wasted money that is flying around the departments I am sure they could have paid a couple of people to transcribe them onto a secure server where they could have just sat until they were needed