I actually still have several old 5.25" drives laying around that I could throw into any of my PCs if the need really arose. When 3.5" drives were becoming more popular, I made sure I had anything important off the 5.25" disks and transferred to the 3.5" disks. Now with hard drive space so cheap, I haven't bothered putting in a 3.5" disk in my latest PC. I have games from the 80s, a few of which don't need any emulator and play just fine on WinXP for me.
But nothing changes overnight where you don't have plenty of warning to start transferring or converting files to the new blank media or new format. And when that change happens, there's always people around who will be coming up with ways to convert formats if there's a need. And if you need an emulator, just Google "DOS emulator", they're still very freely available. Maybe another 20 years from now you won't be able to find it, not the case yet though and I don't see it being a problem for a long time yet.
But epub being basically html, I really don't think there's anything to worry about for ages about ebooks. When changes do start to take place to the hardware needed to read the ebooks, or the format might eventually change someday, the user really needs to look into conversion options.
It sounds like you're looking for a foolproof, idiotproof way to just magically preserve present day formats and hardware forever. That's not ever going to happen. Users still need to be aware and be ready to make changes. And if they're not really computer literate, they can always come to great forums like MobileRead where there's plenty of help walking people through issues they may run into. And if they can't find their way onto the internet to find forums, I suspect they don't even know what an epub is to care either.
But no one can predict the future about where technology might be in 20 years. Other than watching Star Trek and seeing what gadgets are there, they have a way of making it into real life sooner or later.
