Well technically, Spotify doesn't have the option of working with "another real estate agency" since you have to go through the app store to sell to iOS users. The next alternative, simply packing up and leave, is basically cutting the nose to spite the face because Spotify just loses a very lucrative market and leaves a void for Apple Music to fill.
I emphasise with Spotify as I can see that they are caught between a rock and a hard place. They have yet to make a profit and Apple's entrance into the music streaming market can't be good for business, much as they are putting up a brave front now. I still don't think Spotify has a case, but I can see why they would be acting out of sheer desperation.
I am interested to see how this case unfolds though, because while Apple is big (in the profit sense), it actually still commands only a minority market share in the PC and mobile markets, so I am not sure how far you can stretch the "monopoly" card. I mean, Apple has gotten away with bundling its own default browser in every device it ships, in addition to making it the stock default browser and depriving 3rd party developers access to all its features and nobody has sued it so far.