it's also unreasonable for me to expect that Apple should be hosting my 99¢ purchase for the rest of time
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.
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.