"Smart TVs failed to take off" - That's because TV manufacturers cannot write software. Especially with effective and intuitive UI elements. I played with my parent's new Panasonic 50" Smart TV. Jesus christ. Navigating with the normal remote is a struggle, but then try using the touch-sensitive remote they provide. It's so bloomin' sensitive! And there's no logic to what content is where. Just loads of boxes on a screen with labels.
"3D screens failed to take off" - There's never been much of a market for 3D. Only sport/movies have adopted the tech. Mainstream TV (like the Beeb/ITV) will never touch it because of the cost. Especially when their expenditure is being squeezed all the time. I love 3D, but it's a pain for glasses users too.
Mobile TV (via tablets/phones) are the way forward. Once TV makers have figured out that being able to push TV channels via the TV itself wirelessly to mobile devices (to take on Sky's Multiroom/houses with slow internet connections), then people will have a desire to upgrade. 4K will be only relevant to 40"+ TVs.