I think the situation with data migration has improved over time.
My computer use dates back to accessing a DEC-10 via 300 baud modem. I don't have any files left from those days, but it was only a few years later that I realized the only feasible way I had to transfer a file between two computers (one was an Apple IIGS, I forget now what the other one was, might have been a Commodore 64) was to kermit the files across said 300 baud modem to a mainframe and then kermit them down to the other computer. Ugh.
I still have files from the Apple IIGS, both text and graphic, converted to readable formats. Fortunately, the computers I owned in the intervening times have each been able to read the physical formats of the files from the previous systems, sometimes with extra software help, and I've been forwarding all my content as I go. (Also fortunately, the oldest files are quite small compared to the stuff created by modern applications.) But one of my most difficult experiences recently involved getting some data off of a couple of old 400Kb single-sided floppies that I found and had forgotten to forward during earlier iterations... I found an old Mac in the house that could read the disks, but then it didn't have ethernet or the ability to burn CDROMS. There was too much data to fit on a convenient number of floppies. I've forgotten now how I solved that problem. I think I used a friend's zip drive to get the files to a PC, then transferred them via the house network to the Mac, whence I backed them up onto CDROM.
But the past couple of computers I've gotten have been Macs, and they have this great feature where you attach the new mac to the old one via a firewire cable, and it sucks across all the data and applications from your old mac. Cool! (Though it missed all the stuff in the mini-website-- I had to go back and copy that manually.) I just hope Ubuntu or other Linuxes start to offer comparable functionality, as I'm starting to wonder if I want to stay with Apple, due to costs.
To try to give a point to this long-winded ramble, it used to be very difficult to transfer data from one system to another, but the physical and digital formats are starting to be more interchangeable, there's lots of conversion software available on the internet now even for very old formats and orphaned software, and I think if one is willing to keep migrating the files forward, it should be possible to retain data for much longer than the life of any of the individual technologies. That gives me a lot of hope for ebooks.
(And if nothing else, one can always save data to a website and hope that the Internet Archive catches it for you.

)