The killer app for the big screen was and still is to enable immersive shared experiences for real-time events, like sports and various other live performances, and for long-form story-telling (ie. movies). Smaller screens, except perhaps VR goggles or some distant-future holographic room, will never compete.
As other devices with displays take over the short form functions that don't demand that immersive experience, like talking-head news and low-resolution amateur content, perhaps the big TV will become a niche device, only for those who can afford to set a system up that rivals commercial theaters. But I predict that many will still choose to have a near-theatrical experience in their homes.