Backwards is as Backwards does, my Momma used to say
Look some of the files used in the 80's versus the storage mediums. MIDI files are still easily playable, but you'd be hard pressed to find a cassette drive, 5.25" floppy, 3.5" floppy, zip drive, etc. to use on a modern PC/Mac.
It's all about the form factor. Despite Steve Jobs' personal best efforts, the optical drive is alive and well and the form factor lives on in Blu Ray. This particularly form factor has been unbroken since the early 1980s. I can play a 1984 CD on a new Blu ray player as a result. That disc form factor went from 650MB to 50 gig, and has room to grow on as they add more layers (test labs are up to 100 layers). Yeah, yeah, it is all going to die someday -- so won't we all.
As for the protected m4p files, as long as Apple keeps the authentication servers active, I'd bet that you'd be able to play them even 50 years from now.
I think there is a good chance Apple will turn off those servers in less than 5 years. 50 years? Not a chance. Why? Apple doesn't need to give a reason---just because. Maybe you can buy it all back again from them at a one time price, but Apple will turn them off. It is what Apple does.
...
that same computer could probably scan a CD found in someone's basement and play the files back as well ...
As the crew of Monty Python might say, "I'm Not Dead Yet". Of course, if you say something twice, it makes it twice as true, and we've been hearing for *years* that optical is dead, so it MUST be that many times true! RedBox is stupid all the way to the bank with those dead DVDs.
As to file formats, thanks to the sunset of Classic and now Rosetta, they are dying like flies. I can read a floppy disk way easier than I can read some of the files on it because it is becoming hard to run programs that used those formats. My physical media and the playback are way outlasting my ability to open them--and that is for truly dead media, not imagined/desperately wished for.
We tell us ourselves it is the cost of progress and maybe that is true, but like reading Lotus 1-2-3 and old versions of Word or MacWrite, there is nothing you can do if a company stops supporting your specific format/codec, etc---and saving digital media to a different format is lossy (and spreadsheets/documents lose formatting, sometimes badly and irretrievably). It gets worse each generation of the MacOS.
Taking just codecs, it has already happened. I used to be able to edit/save MPEG2s with ease. It is getting very hard to do that now and even iDVD is dead. Buggy as hell too due to neglect. Sure MPEG4 is better, but MPEG2 used to be the best and greatest, and now you have to jump through hoops. It can and will happen with MPEG4. MPEG5/6/7 etc will always be better---and conversion is always lossy---you lose information.
So far my CDs/DVD have had longer life support than some of my old digital movies/audio. Apple can and will sunset support for certain codecs. Next I expect I won't be able to do MPEG2 playback (snap! too late! My iPad doesn't do MPEG2 at all **right now**). Remember Mpeg level 2 audio? I do and I can't find squat that runs those and they sounded great! I have some Quicktime movies that I can't play back on my Macs. I hate Flash but we should add it to the list; Apple wants it dead too.
All of those dead digital formats are way newer than my copy of "Rolling Stones Greatest Hits" I bought in 1989 and which still sounds fantastic on a $20 Walmart CD player and ripped to 256K AAC on my iPhone.
Going all digital is not safe either.
