But really, how often are people offline? For me it's only when I'm on a flight, and I can prepare for that by using spotify or youtube and pressing the download button on a playlist.
I am when I am hiking, camping, or have poor reception like in my apartment complex. There are vast swathes of the globe that do not have connectivity at all.
Also, worth considering the online service could change its offering or die at any time. Songs I like are routinely removed from various online platforms, and it enrages me every time.
I agree with you with content changing being one of the many downsides of relying on streaming services. I just think that most people are willing to accept the trade off.