They burn $50m per year and have 60m readers
They need 1.4% of readers to subscribe
They charge 1/3 what the NYTimes does. The NYTimes company has +$1bn revenues.
They can probably get 1.4% of readers to subscribe. They could also charge subscriptions for blogger tools. They could have tiered subscriptions ($5 is the lowest guardian membership, and it looks like you get little for it). They have only just started offering subscriptions so I imagine what you get for $5 will only grow.
150 people really isn't bloat. Stack overflow has 250 employees and I bet you never paid them a penny. SO have VC funding to series D and they have fewer users and readers than Medium.
It seems that Medium has a viable business model: ads and subscriptions. Same as any other media business.