We all know physical discs are on the decline, even if slowly. So it could be safe to assume that the next PS and Xbox doesn't come with one, though it might, whatever. One thing MS might actually have a head start in, however, is their digital library with 360 and XB games on their current ecosystem. Sony could theoretically do that with their PS1,2,3 and 4 libraries on the PS5. PS4 obviously being the easiest to implament since it will share the same architecture.
Imagine a store like Steam on the PS4 with access to any PS classic you ever played? It would have more games than Steam (or more good games at least). I know a lot of you might say you don't bother with backwards compatability, but don't even think of it as that. It will just be a lexicon of PS games available all in one place any time you need them. I just think having to rely on legally iffy emulators to play old games is quite stupid when in every other medium almost everything is preserved for you to watch or listen to. Gaming needs to get there.