I have a pretty large collection of movies on HD-DVD and Bluray.
When I wanted to RIP the HD-DVD Movies (the original hardware player is from 2008 or so and unbelievable slow for today standards) only 25% of the movies could still be read.
I heard that for example ALL Warner HD-DVDs have rotted, because they used certain chemicals which changed when being in contact with air.
There are also some bluray movies which have problems to be read.
There were also PS4 games I had problems with, despite having used them rarely. I think the killzone disc I got bundled with my PS4 back in the days stopped working.
So no, disc rot is not a phantasy, it is real.