It could even incentivize actors to consider other options, like selling access to download the files themselves
That's fair so long as it was made clear at the time of purchase that you aren't buying the song, and that you're only renting it for a while. If they didn't put a clear explanation that you might lose access on the page when you clicked buy, and not buried deep in a 50 page EULA, then it is entirely reasonable to assume that Apple can afford to keep a 5MB file available for streaming forever, or until Apple closes down its streaming services.
Not necessarily. What if the store didn't have to do that?