There was a lot of hand wringing when they got rid of the DVD drive as well, and now most software and movies are delivered via download.
Having a copy of your own music file just doesn't make sense like it used to. You can stream just about any song from anywhere at any time from any service, not just iTunes/Apple Music. And if people are really still clamoring to be able to download music files, you can bet a service will pop up that will provide it.
Actually, RIAA has always been against letting users download music. The reason they gave in was due to customers flocking to P2P and other illegal music sharing services. They went kicking and screaming into that realization and later acceptance with Apple because making some money was better than no money at all from music downloads.
RIAA would love to see the end of music downloads because it would put them back in complete control. Even better, if streaming services help diminish the need and/or desire for people to download an actual copy of the file, they would be absolutely thrilled. If Apple ends music downloads, RIAA may start intentionally pushing up their per-song rates to drive other music download services to do the same.
While streaming is gaining in popularity mainly because of long time services like Netflix (videos) and spotify (music) becoming so popular, there are other variables, internet availability and reliability being two major issues. While people in most larger cities have no issues with internet availability, it is still more problematic as you get further away from those places. You don't want to be listening to music and it constantly stop or pause while it waits for data reconnection over blank spots or areas with no data access. Bandwidth usage is another problem for a lot of users. Most mobile users do not have unlimited data so bandwidth creep happens with constant streaming. While some carriers offer unlimited data, those plans tend to be more expensive, especially if you have a family and those that do pay for unlimited data have their speed throttled after a certain point.
My main issue with streaming is that due to licensing and exclusive deals with various media associations like RIAA, MPAA, etc., movies, music, tv shows, etc. will get added and removed from streaming services. As popular as Netflix is, I hear people griping because they were halfway through a TV show's seasons only to have Netflix drop it from their network. I don't blame Netflix as I'm sure they have to balance adding new shows and removing old shows, but still, it is quite frustrating to users. If you buy movies, tv show seasons, songs, etc., they are always available on demand without and limitations based on internet access or media availability. Even better, you can watch and/or listen to those as many times as you want and the bandwidth usage was only for the first download of each file.
While a lot of people will never have to deal with those issues when streaming or they may not be bothered enough to worry about it, there will also be a lot of people who won't be happy over the transition toward streaming.