Earlier quoted context omitted.
Pen and paper doesn't have many failure modes. Put it in a fireproof safe, and it will outlive most digital media. No hacker in a foreign country will steal it, no one will encrypt it, no cloud provider will lose it, and no government will read it, no SSD will byte-rot it. There's really not much to be said against pen and paper.
Fireproof safe's, aren't. A thief could steal it out of your house, you can't access it on the go, it'll get water damaged and mold and rot. The ink will fade. You won't have a drive to read the floppy disk/CD-rom/DVD/USB-A flash drive. You can't grep it and unless you meticulously organize it, it's a jumbled pile of unopened envelopes. Cloud and digital isn't the be all and end all, but there are plenty of problems…
I suspect that many people associate durability with electronic media simply because it becomes obsolete so rapidly that we don't experience it in the same way. We don't know whether that dusty floppy disk or CD-R is readable since we disposed of our last computer with a drive, or we associate the failure of a flash drive with a cheap drive rather than the intrinsic properties of the media, or we attribute it to abuse. We rarely have the opportunity to use something until it is worn out, or pick it up after a quarter century in storage.
Modern media is incredibly fragile. The chief advantage it has (in terms of longevity) is its digital nature, in our ability to reproduce it exactly. Yet that is only an advantage if we actively maintain our archives.