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You speak of Microsoft Word as a format that will be around forever. That has certainly not been the case for me. I have some Microsoft Word 4 files (ca. 1995) that current (or even 10-year-old versions) will not open. Those documents are gone forever. (I could revive them, but I'd have to assemble an old-enough Mac and a copy of Word 4, Word 5, or some adjacent version.
I guess you could say that .docx files are okay because they're XML and the actual text is inside them, unencoded. But getting the text out is a nightmare, even if you know how to process XML.
Computer companies no longer guarantee absolute backward compatibility. Rather, there's a sliding window of compatibility centered around an unknown midpoint.
Text files are as close as you can get to eternal. Even with open-sources enhancers (e.g., Markdown), you are at the mercy of the upkeep of tools that convert your enhanced text to its desired more-readable format. Think very carefully about how likely it is that anything more sophisticated than a plain text file will exist in 20 years or more.
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