Apple leapt into the "whole different ballgame" market when they made an MP3 player, then when they launched the iTMS, then when they added movie downloads, then when they launched a cell phone... The natural progression for for video distributors (whether they are cable channels, broadcast channels or streaming services) is to start with licensing third party content and them moving to create their own content. It differentiates them from competitors and provides a more secure revenue stream (no worries about losing the content after the licensing deal runs out).
Dollars to donuts, if Apple started producing original content on par with HBO, Amazon, Netflix, etc., AppleTV sales would sky rocket. But, as we've both said, producing hasn't historically been Apple's MO. Even the long-rumored skinny bundle would help out. With the iPad and Watch not exactly doing gang busters it might be time for Apple to put more weight behind the TV and move it out of 'hobby' status.
As you noted, the majority of people have some flavor of Windows PC and an Android phone so how does Apple get those people to buy AppleTVs when they are outside of the Apple ecosystem? To do that they'll need to offer things that people can't get via other streaming services (beyond just the usual exclusivity window). iTMS was the 'killer app' for the iPod and that, plus taking it cross platform, is what allowed the iPod sales to explode. The iTMS in it's current state isn't a killer app for the AppleTV, and maybe it never will. I'm working under the assumption that Apple wants to really compete with streaming services and cable TV providers but maybe they don't.
The majority of people in the US that will mostly likely buy an iPhone have already purchased an iPhone, and the growth in China hasn't panned out which is why Apple's sales are slumping. For better or for worse Wall Street rewards growth and after many years of amazing growth, thanks to the iPod, iTMS and iPhone, that ship seems to be running out of steam. All successful products run the same basic cycle which is why companies are always looking for the 'next big thing'.
Apple's strength is the quality of its operating system, reason why their overpriced hardware is used / bought.
If Apple focuses on creating content and not improving their current hardware, which the most recent MacBook, iPhone, iPad, Apple Watch, are far, far away from the innovative breakthroughs they use to deliver, then the "need" of using their hardware is going to be diminished, affecting the core business.
Apple's next big thing should be in the consumer electronics industry. The next big OS, the most advanced laptop, the even smarter smartphone or a whole new different thing we haven't thought about (The old Jobs days). Apple revolutionized the music industry without being a record label, and revolutionized the mobile phone industry, without being a carrier; no need to attempt to "revolutionize" the TV industry, doing what EVERYBODY is doing.
If Apple's next big thing is content, following the steps of Netflix or Amazon, the company is doomed; Tim Cook is the most overrated CEO.