Now with about 20 years of digital behind me and still access to most of my early work, I say save it all.
Though DNG sounded good, I've hardly used it over the years except more recently to stitch panos and create HDR files, otherwise, I simply leave the raw files as is and most of my archives are this. Master files are usually layered TIFF files and derivative files are what ever they need to be for the final use, many JPEG from the TIFFs.
Because hard drives are large, fast and relatively cheap, there's little reason to toss files, other than to winnow out the chaff...
Countless times during the deadline of editing, I've missed both gems and found later that my choice of image was not the best. Having the deep and organized archives, nearly all with adequate metadata is a lifesaver.
An image I created this week required finding images I didn't remember I had. A simple keyword search yielded many more photos than I realized, thus making the final product richer. Had I dumped these photos as "obsolete", to save space or because the format may one day become orphaned, this little project would never have materialized.
However, another factor for "future proofing" one's work is to not only create a master file but to print it on paper since in centuries of use, paper seldom becomes a victim of digital rot and the march of technology.