The K-S1 was announced in August 2014 - a nearly 2-year life is not that short (looks like it was longer than the K-m, K-x, K-30, or K-500). The K-50's long lifespan was unusual for non-flagship bodies.
Interestingly, the K-S1 is still on the products page on the Ricoh Japan site, while the K-50 was moved to 'Discontinued' at least a week before it was announced on PF. I really think the K-S1 was designed for the Japanese market - perhaps it will be available a little longer there.
This does leave Pentax without a metallic blue body ine the lineup for the first time in six years. Time for a another petition!