What you say is true. But if a technique makes it so that 1) they don't preserve a record for the future, and 2) they do leak (or risk leaking) information that can kill service people, I personally care more about #2.
(Ironic that, in trying to not leak to future investigators/prosecutors, they increased the risk of leaking to foreign adversaries. Shows which threat they're focused on.)
The Vice President of the United States cannot use Signal "disappearing messages" to correspond with anyone for any purpose.