Well, the other danger is that the things that allow you to play the library contents may go away. Eight-track tapes, VHS movies, etc.
It is something to think about, but they'll still be around for quite a while, courtesy of eBay at least and probably Amazon as well. I have one on Watch on eBay right now (new in an unopened box). Many older TVs that are still around will have the ports where you can plug in a player. As long as the TV works, you can use it as a monitor.
You didn't mention music cassettes specifically, but I would bet you can still find plenty of players for cassettes, too.