Oh shit, so in order to bring back the dawn/sun... you need to sacrifice the night/moon...?! Lucasian poetry.
Yeah that's basically the thing. THe lucius line is inherently evil, and represents darkness. THe oracle line represents light and goodness in the world. Without the oracle line, the darkness that is the Lucis line will throw the world out of balance and infest the whole world, hence the plague of stars.
Without Luna dead, the oracle line was stopped, Noctis had no choice but to off himself and cancel out the plague of stars