I wouldn't conflate demand for USB sticks like the Amazon Firestick or Chromecast with demand for set-top boxes.
Apple TV slipped to fourth place in this space this year and all set-top-boxes (and sticks for that matter) are on borrowed time while Smart TVs improve. I'm not saying it won't move some units, but Apple TV is not going to be the market disruptor people are assuming. Convergence is a powerful thing.
Why? In terms of functionality, they are basically the same.
Roku 3 and the Apple TV box and the Fire TV box work pretty much the same as the Fire TV stick and the Roku stick, just a little faster. The sticks are cheaper, so it's not surprising that they've sold better.
It's possible that if Google could somehow get every TV manufacturer to use Android TV to power all their TVs going forward, then yes, perhaps Apple TV could be crushed. But so far only three TV makers have signed on to use Android TV and they represent less than 10% of the market.
Samsung and LG and Vizio and all of the big ones are still using their own individual platforms. There's tons of fragmentation and while some might be decent, most of them aren't. Even if your smart TV works well, you're not going to get as many apps.
In 3 or 4 years there will be thousands and thousands of apps for Apple TV. Some you can't even concieve of right now. Will your Samsung TV get them? Maybe. Your Vizio and your LG webOS TV and all the others? Probably not.
Then what about upgrading? Maybe the apps need better hardware? Samsung has their evolution kit idea (which notably costs about the same as the rumored new Apple TV), but what about the other TV OEMs? Most people wait 8-10 years to replace their TV.