Fixed costs isn't the cost of having a single server with the storage. I'm thinking everything they need to have built up with the intent of having between N1 and N2 MAU, in order to make that viable.

It's the cost of developing the software stack, of hiring the lawyers and accountants that (1) acquire the music rights and (2) handle the music payouts, it's the lawyers that handle the different legal requirements across every major global economy, it's the servers located in all of those countries with as many sub-national locations as necessary, it's the IT staff that manage that server uptime, it's the software developers that maintain that system and improve upon it so rivals don't jump too far ahead... Etc.

Building a streaming platform that expects to have multiple billions of dollars in revenue across hundreds of millions of users is going to have enormous fixed costs that cannot be trivially scaled down if user counts are lower. If they plan around a much lower user count they can scale it down at that planning phase, but not after the fact (at least not easily).