Spotify had a large headcount.
Let's look at some of your examples and see where things can logically be cut.
>I imagine there's a big team dedicated to getting inventory on the service through negotiations with music producers around the world.
This can be streamlined considerably through standardized contracts. I'd wager 90% of the inventory that said team gets is from niche artists, independent artists or small labels. Very few artists actually have the clout or resources to negotiate their own deals with Spotify. That's sort of the point of having executives - to handle the Taylor Swifts of the world. Your local band, rapper, singer-songwriter or producer doesn't matter to Spotify.
Have a clear and straightforward tier system for compensation. You get a billion streams a year? You get $X. You get 10,000 streams a year? You get $Y.
>There will be a large editorial team producing playlists, curation, tuning the algorithm with expertise, etc. That all needs to be localised and to cover lots of niche music tastes. That's also not evergreen content or work, it needs constant updates to remain relevant.
You don't need a large team to curate playlists when you can have users curate playlists. In fact, they already do. You don't need a huge localization team either. Users already take care of the legwork for you. Same goes for algorithm tuning.
>Then there's international expansion and enabling the tech to work for more countries, more languages, more payment methods, more types of music publishing.
Spotify is kinda already there in terms of international expansion. Anywhere they aren't already, they probably won't get there for political reasons (such as China).
More languages? Sure, but that doesn't require a massive team. Fortunately, there are a few dominant languages around the world, and Spotify has those covered.
More payment methods? Let the creators of those methods handle that - that's their job. Credit Card companies or banks or whatever don't create their own payment platform tech and just say "now it's yours to try to implement", they have entire teams dedicated to that.
More types of music publishing? Like what? There's only so many methods of publishing music, and I'd wager there's going to be very little (if any) innovation there for a while. Spotify doesn't really distribute any other forms of published music besides digital streams. They aren't selling records or CDs, are they?
EDIT: you could also say Spotify has vastly overspent on the likes of Joe Rogan, no matter how popular he is. $200 million dollars to a company that isn't very profitable, no matter how large, is just bad business. He's worth nowhere close to $200 million.