I bought the original model (Series 0) and used it all the way up until I got my cellular Series 4. A combination of OS updates and an aging battery forced me into it as it was becoming so slow that it was borderline un-useable, and was seldom able to last through the day without going into power reserve.
It's now time to replace my cellular Series 4. It's still as snappy and responsive as ever and isn't struggling with any of the newest OS features, but the battery is not lasting the day anymore. It could be the OS is hammering on it harder these days, but it's more likely just heavy use. I use my Apple Watch as an iPod, a GPS unit, a phone, a messenger, etc. I love leaving my phone at home when I'm exercising or just doing something where I don't want to have to keep track of my expensive smartphone. I can totally get by with an Apple Watch and my AirPods Pro in those situations and I do it often. Totally worth the extra few bucks on my Verizon bill.
So I'm not unhappy with my almost 3 year old Series 4 at all--I'll be upgrading more for a new battery than anything else. I could probably even squeeze another year of use out of it if I really had to, maybe even longer if I sprung for a fresh battery. I would say you could get a good 4 or 5 years out of any given model going by the current pattern. Once they majorly refined watchOS, the hardware became more limited by battery age than anything else.