You're not being asked to care about what I or anyone else does with their first generation Apple Watch. The point is, it is not being obsoleted. Owners of of these watches can keep using them or not entirely on their own volition. The same will be true when the next generation becomes unsupported by OS updates. It is neither unusual nor the end of the world as we know it.
Of course they can, but they will start missing out on a lot of cool stuff.
Here's a good quote on twitter from _davidsmith (who has written loads of Apple Watch apps):
"My favorite announcement of the morning. Goodbye Series 0 Apple Watch. It’s been fun but you were holding the platform back. I can do so many more interesting things with Series 1/2 was the baseline! https://t.co/W2olzP16G7"