I don’t think expectations are low in the way you suggest. We have many users who are still using much older devices without major issues - and also some who get problems within a few weeks of buying a new device.
As far as your battery life goes, Apple rates the watch batteries for something like 80% capacity remaining after 500 charge cycles, and that is governed more by the chemistry of the cells than any warranty consideration. It depends how you have used and charged your watch but at 18 months old that is around 500 days so your battery could be approaching the point where Apple’s judgement would be that the battery is at the end of its service life. At that point it becomes a matter for your decision whether the cost of repair is economic, or is complete replacement a better choice.