>>woodru+(OP)
This is what I would have guessed, or otherwise it would be hard to explain why e.g. YouTube stuck with a purely ad-driven model for so long. Also subscribers usually get unlimited content for a flat fee whereas ads-based users pay incrementally for bandwidth and resources consumed.
I think it's possibly useful to think of the revenue streams as a form of diversification. For example, there's value in Google diversifying some of their dependence on ad revenue.