I would be equally concerned about the stability of the file formats for the data stored inside the archives. Even plain ASCII text files have not been around v

I would be equally concerned about the stability of the file formats for the data stored inside the archives. Even plain ASCII text files have not been around very long - about 60 years since standardisation, but it took a while for the standard to become largely universal. And ASCII is pretty restricted in what it can represent. Note that I'm talking about plain text files, not things like Markdown which might use ASCII. Most more complex file formats suffer from variant formats. Some, like Markdown and RTF just have multiple versions. Some like TIFF and PDF are envelope formats, so the possible contents of the envelope change over time, introducing incompatibility. Then there is bit-rot as formats go out of use, e.g. .DOC (as opposed to .DOCX). My own objectives are simple compared to your brother's. I want to preserve simple formatted text files until about 40 year from now, in a way that is likely to allow cut and paste. I started accumulating them about 20 years back. Note that this is before Markdown (which is in any case poor for recording formatting). LaTeX was around and seemed ok in terms of expected lifetime, but is poor for cut and paste because the rendering of a chunk of text depends on instructions which are not local to it. I settled for RTF, which this carries significant long term risk for both compatibility and availability, but is documented well enough that migrating out may be possible. That's just formatted text. Images have been worse, particularly if you are handling meta-data such as camera characteristics, satellite orientation, etc.