I'm still rocking my Series 0 in silver stainless steel. Looks and works great still. Its battery is starting to show signs of age but there are shops that will replace it for less than $100, if I choose not to upgrade it.
Based on my own consideration, I'd say your question boils down to:
- New features. Per your post, you upgraded after six months because of a new feature you wanted, not for any lack of performance. That's not a criticism-- if you can afford it and make use of the new features, go for it. Apple is constantly developing compelling new features. The advancements in the Watch versus my venerable Series 0 are startling. I haven't upgraded yet only because I'm cheap. But as my battery starts to show its age, I'm keeping an eye out for deals.
- Software support. Updates will eventually stop coming, as they did for my Series 0. But, note: it works just fine. There are just updated apps I can't have and features it doesn't possess.
- Physical condition. The stainless steel models with their sapphire glass are almost indestructible! I'm tough on watches, and my previous titanium Casio (which cost about as much as an entry level Watch) looks like it's been through a war. Not my Series 0. Looks like new still. When I replace it, I might go for the aluminum model just so I don't feel so guilty upgrading for new features a couple years hence. Not only would it be less money, but it'll look like it needs replacing. If I get another SS model I'll feel traitorous replacing it before it fails!
So "good performance" will probably not be the gating item for your decision a year or three down the road. And that's just fine. The Watch will be performing well. But perhaps you will value other aspects of a new Watch at that time. There's a reason Apple Stores see folks lining up days before a major product release.
Incidentally, some have griped about the Series 5 having insufficient new features versus Series 4. They note the extra storage and the always-on display, but often missed is the 5's cellular model's new ability to issue emergency alerts even if you don't activate it, and even in most international travel. That's a huge benefit! And a justifiable reason for a Series 4 wearer to upgrade, IMHO. More info:
https://support.apple.com/en-au/HT206983 and
https://www.apple.com/watchos/feature-availability/#branded-international-emergency