Series 0 here, waiting for it to not be supported by the newest WatchOS (or for the battery to wear out, whichever comes first). Whether that means I'll be buying a Series 3 or 4 or 5, I don't really mind.
Given Apple's history with updates, I think WatchOS 4 will definitely work with the Series 0 watches, but it will be slower on the old watches (because of lack of the dual-core processor), and not include certain features that would run too slow or use too much battery.
And there will be a few new features that will only work on the new Series 3 watches that come out, my guess, in the fall.
It's not that Apple tries to kill off support for older devices, despite many naysayers... it's just that Apple wants to move ahead. Yes, it helps their bottom line, but it also improves their newer products.
Apple made WatchOS 3 to work fine for all models by not including many features that really kill a one-processor Series 0. Now, they will move ahead, and there will be all sorts of new features that use many background processes, which will really need a dual-core processor to work.
One thing that really works better on Series 1/2 right now is Siri. It never really worked, was too slow on Series 0,
that I could never use it. On a Series 1/2, I simply raise my watch to see the screen, say 'Hello Siri', it responds immediately, and I can ask what I want, and it comes back with a reply in seconds. There must be a background siri process running on the Series 1/2, which makes this whole thing smoother and work.
I bet there will be a feature, like 'always on', where the watch face is always on, but Apple will restrict it to Series 1/2. It's very obvious why. My old Series 0, could barely last an active day, many times getting to under 10% at 18 hours. My Series 2 lasts TWO days. Apple doesn't want the Series 0 dying at like 8 hours with always on, as they'll get all sorts of bad press from Apple haters. So, that's why they restrict it.