Apple has issued guidance to developers, suggesting that it will discontinue support for watchOS 1, with updates for apps limited to that version no longer accepted after April 1, 2018.
Apple mandated that new apps for the Apple Watch use watchOS 2 in June 2016, so the change isn't any surprise. Thursday's mandate applies to updates to older apps.
Newly submitted apps must be built with the watchOS 4 SDK.
In the developer's note, Apple also points out that the watchOS bundle size has grown from 50MB to 75MB.
There is no technical or performance reason for original Apple Watch owners to still be on watchOS 1. Improvement and enhancements to the operating system since its debut have made the so-called "series 0" device faster and more stable.