I'm not going to try and discredit any of your opinions above, I think you have a lot of valid points. However, when it comes to realism in fiction, you have to consider the notion that audiences will accept almost anything as long as it has believable world-building. When fictional universes break or expand their own established world-building in a way that's not consistent with previous titles, there always tend to be divisive reactions.
I'd say that story-wise the Resident Evil franchise has its roots in science-fiction horror, and not the supernatural horror that's been seen increasingly in the later games. The most prominent example of this is actually Eveline in RE7, whose supernatural powers seem so far off the table compared to the lab-created viruses from RE1. Come think of it, the original Japanese name for the series is BIOHAZARD, which in itself is much more of a sci-fi horror title. A very good comparison would actually be Jurassic Park, which is labeled as science-fiction horror. In Jurassic Park, we get to see dinosaurs interact with humans which is completely fantastic, but the idea that's its created by scientists is a believable explanation and that makes us think that it COULD happen in real life. That's science-fiction horror, and the same can be applied to the original RE games.
In my opinion, RE1-3 are the most realistic games in the series because they deal with a virus infection created by scientists and a cast of ordinary people trying to stop it. The RE games started to jump the shark with games such a Zero, when we are introduced to leeches being controlled by a hive mind that sings opera. At that point, the science-fiction elements of the previous games are no longer believable to explain such things, and they start to become more supernatural. And when things become supernatural, it's no longer something that makes us think it could happen in real life. Unless you believe in supernatural stuff, which is stupid.