>>96492844
>there's no Mortarion or Maugan Ra without the Nightbringer
This is what the original Mortarion model looked like. GW made this because they thought that the daemon primarch of Nurgle, a god of decay and disease, would look cool if it was the Grimreaper. Which isn't surprising because the modern renditions of the Grimreaper specifically originate from the Black Death. It's the reason why modern version of the Grimreaper carries a scythe, it was to symbolize him reaping the souls of the dead, the people who were dead due to the bubonic plague which killed millions.

And this model was made long before the Nightbringer was ever a thing in 40k.
Nightbringer is from 3rd edition.

Maugan Ra on the other hand looks like a grimreaper nowadays not because of the Nightbringer, but because he is the Phoenix Lord of the "Dark Reapers" aspect. Aka an aspect which already references the Grimreaper.

GW designers are lazy, and Necrons aren't the blueprint for the other humanoid races.
They never were.