The marginal cost is only zero within each size category you exist in. It scales as you grow readership in strange ways. An additional 1000 subscribers might

The marginal cost is only zero within each size category you exist in. It scales as you grow readership in strange ways. An additional 1000 subscribers might mean you need to spin up an extra VM to handle small loads. That's fine, still have marginal costs close to zero. At 10.000 more subscribers you need to diversify your coverage. More content producers cost money. 100.000 more subscribers some critical infrastructure breaks unless you employ sysadmins to manage it. And so on and so forth.