The problem with it becoming the tipping point for stream vs d/l is the data use on a mobile device. If you stream music regularly you will really hit data caps or need a larger cap, unless you have wifi access. In addition, it means no music when you can't stream, such as when flying unless you want to pay for wifi where it is available.
I'd like to see a two option model: buy the song outright or be able to stream and download with the d/ls only playable while you have an active subscription. They could auto-expire after a certain number roof days and whenever you connect to the server automatically extend the date to your next renewal date.
What you're asking for, Google offers. Of course I think everyone here will complain that Google will track your music listening, and Apple doesn't.
So if Apple's smart, they will offer the ability to download albums for use offline and that'll be fine as long as your subscription remains active.
I love Google Music All Pass by the way. I can use it on PC, Mac, iOS and Android devices.