The first Apple Watch, the Series 0, can’t run watchOS 5, 3 1/2 years after its introduction, 2 years after it was discontinued. Has Apple made any statements about whether there will continue to be security updates for wachOS 4 and if so, for how long? My recollection is Apple doesn’t do security updates for previous iOS versions but I think they have done so for Apple TV software (i.e. an update for the 3rd gen.'s OS after the 4th gen. and its new OS was out).
I’m also wondering if watchOS 5 will be the last for the Series 1. It was introduced in 2016 along with the Series 2 but with a slower processor and worse GPU; the Series 1 is basically the same as the 0 but with a dual-core processor. The Series 2 was already discontinued last year, when the Series 3 was introduced, and the 3’s hardware was significant step up so it’s plausible that watchOS 5 will be the last for the Series 2 as well.
I have a Series 1 and don’t think it needs continuing major updates but as a very connected device, the Apple Watch should have at least security updates for a decent length of time. 3 years after a model’s discontinuation (i.e. the latest time an AppleCare hardware warranty could cover) would be good and would only require 1 year of security updates for watchOS 4 to fulfill. If watchOS 5 is the last for the Series 1 (and maybe the Series 2), it would need updates until Sept. 2021, 2 years of security-only updates if a watchOS 6 comes out in a year.