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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.
I use DEVONThink and Tinderbox as well, but there’s one tool I’m increasingly relying on that is not here yet more Zettelkasten-friendly than both combined! It’s Obsidian (I have no affiliation with it). You can configure the settings to automatically parse notations in Zettelkasten format, such as fronts and dates. It may look intimidating to inexperienced users at first but I watched a video tutorial on YouTube called Linking Your Thinking about it, and was on my way.
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