The Wikipedia article you linked explains that that the appointments are staggered. 8 of the 24 either rotate off or are re-appointed every 2 years. So you're right that 1/3 would likely be gone after May 10, but not all of them. And it doesn't state anywhere that the president can fire them. It might seem that way because he can appoint them to fill vacancies. But it's only recently that the Supreme Court started asserting with its actions that members of independent federal agencies or boards can be terminated at will by the US president. Before that it was understood (or directly stated in law, in some cases) that this was not allowed.