If the elite only allow morons morons to live, things would presumably stagnate and become uninteresting for them. Society would just devolve into an ant farm or zoo. Presumably the elite's goal would not be to kill off people above or below a specific intelligence level, but rather to design a society, composed of both morons and nonmorons, in such a way that maximally elevates their level of power in relation to all non-elites, keeping them in power as firmly and for as long as possible. Presumably, this drastic power differential would maintain the illusion (both for the masses and themselves) that the mortal elites are like living "gods". This seems to be what they are aiming at with the so-called 'vaccines', universal surveillance, transhumanism agenda, etc.

However, it does seem the average intelligence level or bell curve has declined over time, partly due to natural factors and partly encouraged by social engineering. I assume the elite do not need a very large number of intelligent people to serve them. So I agree that the elite would want to kill off most nonmorons, but they also do not need so many of the useless eater morons. The vaccines are targeted at both morons and nonmorons, and this is corroborated by the fact that many high-IQ people have taken them as well as submitted to the broader program.