7 years ago I tried to start a project to allow customers to pay through the ad networks. IE rather than seeing ads, you'd see something saying how much you were providing to the web page owner. I've so far been unable to actually build it though.
Yes, and when I bought CDs back in the 90s I didn't really expect to be listening to them 25 years later. My tastes change over time.
On the other hand I'm a little confused what you mean by the format going away in 5 years. The format is sufficiently open that anyone can write a player for it (in theory). Unless I lose all copies of the files, there shouldn't be any reason it won't continue to exist as long as I like listening to it.
I buy .m4a files from Apple and download them, then copy them from my laptop to my Android Phone. At this point they're so disconnected from the Apple infrastructure, I don't see any way they could pull it out from under me.
I think the server would be able to encrypt with the user's public key, that way they wouldn't be able to read it. They'd have to send the encrypted data to the client to be decrypted with the private key there