That's why there will be converter programs to convert all the data to the new format.
But it never looks as good. Assuming you even realise that its not supported in newer versions of the program.
I have the TSR's Advanced Dungeons & Dragons CD-ROM from 1996. Still works. Can still read those digital copies of the core AD&D 2nd Edition rulebooks. In their glorious .doc format designed for Microsoft Office 4 for Windows 3.1
Wheeee.
And I'm sure people here who purchased some of the original WotC 3e PDFs can tell you all about the
fantastic quality of those PDFs. Low rez, not always OCR, poor cropping of the covers, limited bookmarks, no hyperlinks, etc.
Formats change. Standards change. What I considered to be a great looking PDF just ten years ago looks like amateur hour design now, and probably looks like crap on my iPad.
What we consider a "large" file will change. Compression will change.
I've literally bought some movies four or five different times.
Army of Darkness.
Star Wars. Because the formats and standards change. Because the media is updated.
Why shouldn't rebuying digital books immune?
Luckily, digital data does get old or fall apart and there are infinite copies.
I have a few MP3s that have been copied from hard drive to disk to drive to disk several times that would disagree with you. Copy and read errors happen.
Funny thing, I just copied all my old burnt CDs and DVDs of gaming PDFs back onto my hard drive. So I could back them up onto the cloud rather than physical media. Not every file could be read. And the oldest PDF I have is from 2001. It's still readable, but it's embarrassingly bad looking. Like watching an old VCR tape.