Google has Google TV. It's rancid with ads too. Basically the top half of the home screen is ad space, with more sponsored content recommendations below. There for a while it was running fried chicken video ads on that top half. That's when I decided to replace my ChromeCast with Google TV with an Nvidia Shield that has all the Google crap gutted out of it. Not exactly perfect, but so much better.
Unfortunately, digital rights restrictions prevent an open source option from taking off. None of the streaming services want to allow their content onto a platform they can't easily manipulate.