It would be interesting to know if they earn more money on subscriptions or on ads. If it is the former, then why not cut all free plans and leave only the subscriptions?
If I understood correctly, they pay a fee to the records company each time someone plays a song. Even if they're making half their revenue on the free plan, cutting royalties by something like 95% (just guessing) and losing 50% of the revenue doesn't seem like a bad idea... Of course they lose on advertising, but is there something else I'm missing?