If you don't want to buy a gadget that will be obsolete in a few years, then you don't want to buy a gadget.
Constant movement forward, whether it's in terms of power, features, connectivity, price, etc. has been a fundamental dynamic in tech gadgets for decades. Even quality hardware, that will last a lot longer than a few years, ends up abandoned or unsupported by ecosystems we use, or overwhelmed by software updated when they are supported.
If that bothers you then, by all means, do not buy an Apple Watch. But clearly this hasn't stopped gadgets from growing into a business worth billions and billions of dollars a year.
Fear of obsolescence isn't a reason not to buy an Apple Watch. It's a reason not to buy any tech gadgets.