"In real life" You mean in THEOLOGY. Theology isn't real life. It cannot be proven whether angels/demons and even God(s) exist.
Well. Yes. I don't think /u/Teenageboy18 was trying to prove the existence of God here. They meant in non-tv show terms. "Real life" terms and concepts, when utilized for fiction, can be as true to the "source material" as the writers want them to be - or as far away. So in theological terms, X may be one way but someone may write something centered on X and reimagine it for entertainment purposes. I mean, the show Lucifer imagines him as a nightclub owner who zips around in a sports car and solves crimes...
Following deductive reasoning:
What deductive reasoning? That is the other poster's entire point. You would need to establish what information you are even using. I don't recollect the show ever referring to Lucifer as a demon, and the fan wiki doesn't either. It indicates he is a fallen Archangel affiliated with the demons, which it defines as "evil spirits which originate from Hell, who seek to corrupt, torment and even possess human beings, and in general to wreak chaos and destruction on earth."
The same way Humans are no longer Neanderthals from a biological point of view,
Ehhh, I wouldn't go this route.
Neanderthals were humans like us, but they were a distinct species called Homo neanderthalensis.
We didn't change species. We're related. How we are related is still a topic of conversation today, not everyone agrees on their classification/description of species and it's still being researched. Plus, at that point it feels like we're talking about possible evolution and I have definitely never heard the word "evolution" discussed alongside demons, angels, or Satan, in the show or in theology. lol