If the property of "io-lessness" becomes something statically verifiable as part of dependency handling, it also seems potentially beneficial as a guard against supply-chain attacks.
... but only on the volume it is explicitly given access to. So, if the library was IO-less (and didn't use unsafe code), you could embed it in some tool, e.g. for forensics, and not have to worry about it compromising the security of the "host" system.
