Several things are hurting (not killing) broadcast TV:
1. Timeshifting. Lots of shows get watched on DVR now, and ratings don't "count" as much when you do that.
2. More time to fill. Broadcast networks have a lot
more slots they need to fill with original programming than cable networks do, which means it costs them more to produce all the shows they do.
3. Mass appeal. This is the biggest difference between cable and broadcast. On cable, a show with half a million to 2 million viewers per episode can easily be a breakout hit. On a broadcast network, those numbers would get you canceled in a hurry. Broadcast is built on a mass market model, and yet these days people want their specific niche catered to, which is what they get with cable.
4. Obscenity regulations may be a small part of the puzzle, but they wouldn't be hurting the networks in any big way. Besides that, the TV rating system lets them show just about anything they want, as long as it's got an appropriate rating.
Scripted programming just may not have much of a future on broadcast TV.
Chromecast just sounds like a competitor to Apple TV, Roku, and game consoles (at least the web functionality/media center part.) It's already a pretty crowded field. Guess we'll see if Google brings anything new to the table.