As Crashfrog has said it isn't as simple as DRM (although there is DRM in the mix it can be circumvented and would be if Apple went away.) In practice it might be inconvenient if Apple did fold, but it is not a straight loss.
The other point I want to make is that from a purely ethical standard HOW you buy the music is not really the issue. The issue is whether you paid for it AT ALL. You should feel free to make backups or copies in other formats - so long as you buy the music somehow. If you have bought a somg from iTMS there is no reason to feel bad about ripping a copy from a borrowed CD.