I think that this can be expanded beyond just "only use plain text for important documents". I think the heart of it is really, "use only proven, open (preferably standardised) formats for important files." E.g. Markdown is a proven format (plain-text-readable but not strictly plain text), so it is likely to stay readable for a long time, and it is an open format, so you can use it right now without paying for proprietary software - so it would be suitable to use for an important file. HTML also fits the bill, so it would be suitable to store documents. (It seems that the "plain text" of this article actually uses HTML heavily.) Or, if you are making vector graphics, save them as SVG files instead of Adobe Illustrator files. And so on.
You've got it exactly right. Years ago, I wrote "Any sufficiently important information must be indistinguishable from plain text", and your point is what I was trying to get at: if it's that important, make sure the format you use has tools that are as helpful, flexible, and degradation-resistant as those available for plain text.
