Really? So let's assume you are earning X Dollars per month, and your employer asks you to take a pay cut for three months, and then you will get royalties which make up... about... X Dollars a month, you would do it?
Your calculation only sounds great, because you make "royalties for every person that uses it" sound great. It's not. Not for the majority of artists.
The fact is that the artists currently get royalties. Apple is asking them to accept less royalties for three months, and after that, they will go back to what they had before, plus a potentially minor increase. Really, you would accept that?
In some cases, yes. This is the basic thought process that goes into refinancing a house. You pay the bank a lump sum of cash upfront to get a lower interest rate on your mortgage. The lower rate, over time, will eventually allow you to recoup your original costs and then you reap the benefits of the lower rate from then on. Albeit, the refinancing analogy has a lot less risk in it as you know that you will reap a benefit in the future as long as you stay in your house long enough. The artist, does not know whether or not people will listen to their music more or less due to the "upfront, lump-sum payment" of royalty-free music for three months. I think most artists would think the risk is far too great.