Not this again
Samsung releases updates to the Galaxy S line between March and May each year, roughly. Their sales are down in the late fall/early winter because their flagship is 'old' by then. The Galaxy Note release in September keeps their supply chain rolling at a hibernation pace in the meantime. It is part of their strategy. It is going to happen every. single. year.
It is not news, it is not cause for alarm. It does not mean Samsung is in trouble.
Additionally, since Android devices last longer, most people do not feel the need to upgrade every year like Apple users do. Therefore, every other Galaxy S device sees significantly higher sales. The S5 was lower, so the S6 will be higher due to pent up demand.
I myself will be looking to upgrade around June/July, 2 years after my S4. The frontrunner will be whatever Samsung phone comes out in the spring, although Moto has been doing good things lately and will be worth a look.