Yeah if you search on the Apple website it mentions the expected battery life and cycles for each product, and when they consider something replaceable.
With the watch, unlike laptops, they can't just change the battery so they have to give you a new piece of hardware to replace they battery.
Weird story, my AW Series 1 started chewing through battery on a daily basis, and after a trip to the Genius bar, there was a sensor that was malfunctioning. So it was causing the watch to search for it constantly and was eating up battery life. For a while it seemed like a software issue but eventually I'd get like 10 hours out of it, max. So sometimes you think it's the battery, but it's something else. I upgraded anyway for water resistance. I'm not sure what the Apple policy would be on the battery even when it got to 80% given it had an identified defect.
On the subject of battery life, I then bought a used AW3 from eBay and still have 96% battery life after 2.6 years, I have it set to maximize battery life and it's still going strong. I can almost get 2 days out of it without charging it. I just bought another used AW6 LTE being curious about some of the other features like O2 measurement and altimeter readings, and the battery is already at 91%. I figure it's at max about 1.5 years old. I had it checked at the Genius bar and everything was fine, but with the LTE and Bluetooth, etc. being used, it can take a toll on the battery. 91% didn't sound unreasonable for life per the Genius for this generation.
If it's just a pure battery thing, I'd say go for it at $79 if your watch has all the features you want/need. The only thing I see for potential future changes is a better battery life. Honestly too, I was happy with the 42mm watch size. Just big enough, but the right size for my wrist. If it gets any bigger I'm more than happy to keep my old watches.