I know no one can answer this for me, but something worth thinking about...
I love how we've had iPhones that have been broadly the same shape forever, iPads that have been the same shape forever, Macs that have been the same shape forever, and yet every year people start panicking that the Watch is going to significantly change and lose compatibility with existing bands.
If you look at how Apple develops products, the mantra is clear, tried, and tested. They start with the core product. The next few generations add the main big things, at the expense of weight and thickness. The iPhone for example got thicker as 3G was added. The iPad got thicker and heavier as it got Retina. The Watch got thicker as it became waterproof and faster and had GPS added.
What happens after that stage is refinement: making things thinner and lighter. We saw that with the iPhone, iPad, and Macs.
This is the Apple way. Or at least, the Ive way.
For Apple Watch, we've just hit that refinement part. The core product is there. And there is plenty of scope to make the Apple Watch thinner, lighter, and more refined without changing the band mechanism.
Just my two cents.