I do have devices that can read tape written 20 years ago. 8-track was from the 60's and 70's by the way, making it a bit more than 20 years.
First CDs came out in early 1980's. That's 30 years and the drives are still backwards compatible -- I can still put in my old Pink Floyd CD into my Blue-ray drive and the computer will play it. There is no reason to believe that won't continue for another 10 years or so as music is still being released on CD in commercial quantities. Despite the hype optical disks are far from dead.
Archival quality optical disks have a good chance of continued support because there is a substantial effort being put into them in the background by people like the National Archive, Library of Congress, and others looking for means to maintain long term records storage. It's not guaranteed that they will be around in 100 years. Maintaining any system that long is difficult, even simple paper. It's got as good a shot as any though.
What this country needs is a good five dollar plasma weapon.