It's a first generation device for Apple. The first iPhone had to be plugged into a computer. This watch is for early adopters, you know the 10-50 million people who'd shell out the money.
The watch's abilities will greatly improve over time, thus becoming more useful to another 500 million people.
At that price point, smart watches will remain a very niche market. Just as niche as $600 watches are today for consumers globally.
Android will end up dominating smart watches. They'll be made in China, and they'll cost $50-$99. That will drive adoption in the hundreds of millions globally.
As for the size of the market, I'd expect almost everyone on the planet to want a wearable when you can use one to better monitor your health (heart, blood pressure, sleep, etc), use it as a credit card, remote for lights and garage door opener, key for house, etc.
7 billion people, 7 billion wearables.
http://www.worldometers.info/world-population/
8 billion by 2025.
That sounds like a very ill gotten theory. Do companies really expect people to go out and by accessories that cost in the hundreds and add more stuff for people to hull around that do roughly the same things?
No, If I'm buying a smart device it's because either the cost is so cheap it's like buying extra USB cables or it's something to replace my other smart device.