How long was the original iPhone useful? It got OS updates for several years. But even today, I think all it does or did still works. It still syncs with iTunes. Sure, it did not have many if any Apple cloud services, so, something like iCloud Photo Library might not work in five years anymore if Apple replaces it with something else. But then Photo Stream still works despite being replaced with iCloud Photo Library.Sure? The watch is tightly connected to (and only really useful with) an iPhone.
Will a 2015 Apple watch be compatible with 2020 iPhone with 2020 iOS? What will you be able to do with the watch if not? How long will the cloud services for the watch be alive?
The question is what an Apple Watch can do without an iPhone. It won't be able to connect to the Internet (but that would be analogous to the cloud services of your running watch). Currently, third-party apps essentially need an iPhone connection but I am sure that there is a decent subset of watch features that will keep working without an iPhone in range.I use a running watch that is 4 years old, but its not the same. There are more advanced models now, but the watch still does what it was bought for and it will still be useful, even if the cloud service for it is shutdown one day.