So now, she needs to allow it on Apple Music or she'll look like a jerk. Apple will have this album that no other streaming service has, and they got a ton of press for it.
Probably worked out pretty well for them.
Correction: That estimate was based on the average Spotify rate over the last few years. Apple has now said that the rate will be different during the trial and isn't being made public.
This is about a handful of artists grabbing a little more money, period.
As for Spotify et al, label artists can't opt out. Their contracts won't let them. The reason Taylor Swift's current album isn't on any streaming services is because she makes a ton more selling it and streaming would cannibalize sales. And her current album (and only her current album) she did without a major label... so she's one of the few that gets to make that call.
Streaming services want current music to get and keep subscribers. Musicians want their latest music to be sales and their back catalog streaming (ala Netflix, etc) to maximize income.