How should I make sure that I retain access to today's data 20 years from now?
I still have my long-term MSDOS backups from 1991. The backup file is a whopping 13MB in size, and that includes the OS, a word processor, a C compiler and my source code.
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I just made sure that I continually copied forward the backup files I wanted to retain. Each iteration of archival storage increased about ten-fold, so space wasn't a problem.
I think it is more important to have a good archival process in place. To the OP, the error you made was leaving all the data you want on old floppies and IDE drives, etc.. You should have moved that data off to more current media as your processing moved to more current media.