"Why are apps the future of TV?" Because apps are how you monetize content on mobile devices, and sooner or later all your media will go through a mobile device.

"Why should TVs have a future?" They don't mean TVs, they mean streaming video.

"Why develop another ecosystem just for an larger output device?.. let the smartphone be the TV's brain—and again we talk about Chromecast." Yes. And the software that will control that brain will be written in an app.