The development of the WS-21 engine has indeed progressed somewhat faster than that of the WS-19, though the lead amounts to a difference of only a few years.
In practice, however, the Liyang Factory’s primary production focus for the foreseeable future—spanning many years—will be the WS-19 engine, rather than the WS-21. The WS-21 is currently served by only a single conventional production line, resulting in relatively low production efficiency. In contrast, the Liyang Factory has constructed an entirely new, highly intelligent dual-pulse production line specifically for the WS-19 engine, which boasts exceptionally high production efficiency.
The WS-19 engine is not available for export. This policy is driven by considerations regarding the protection of sensitive technology.
The core engines of the WS-21 and RD-93 are entirely different; they are not directly interchangeable.
The JF-17 can utilize the WS-21 engine, but doing so requires extensive modifications.