> The software analogies to books (documents of various types like PDF, HTML) don't have pathological obsolescence problems, either.

It kind of depends on the document format. PDF tends to display well over time (but there can be issues with complex documents and/or missing fonts), spreadsheets and word processor files are likely to be openable, but they may not display as they originally did. HTML is likely to look rather different than it did when written, but it'll probably be legible.