[LondonViewer]

This is only part of it I think. The streamers are at a point where they have such a hold of the TV market that they can now tap into an incredibly lucrative advertising market that will probably in the long term end up bringing them more revenue than subscriptions alone. Netflix has stuttered a bit with moving people to the ad tier and even filling ad spaces but now they're ditching the basic ad-free tier, and adding more features like being able to use the ad tier on multiple screens, I expect their ad tier will become dominant soon enough. 

Getting subscribers to pay you to watch advertising is basically a win-win for the streamers, as long as the content remains strong enough to keep people subscribing. Not unlike Sky, especially when it was at its peak and people didn't seem to care that their channels were full of ads.

The studios are at a point where they must tap into advertising revenue because the streaming model is no where near as profitable as the model it replaced.