I like this idea with the goo, but it would have to be yet another retcon, because Alien implies the Lv-426 Jockey has been dead there forever - long before the events of Prometheus.
I'm not sure it has to be a retcon. Many of us like to think the derelict has been there forever. We also like the Lovecraftian aspect of O'Bannon's original script. But that's out the window. So, the derelict was apparently a sort of living ship. We don't know how long the it would take to decay on the planet. LV426 has a harsh, possibly corrosive atmosphere. The egg chamber seems far better protected than the pilot's chamber, and the derelict looks more biological in nature than the Juggernauts from Prometheus. Maybe Black Goo did that, maybe not. But perhaps the derelict would not survive for any great length of time there. The ship has earthquake damage in Aliens. Seems likely earthquakes are common. Erosion would be a problem. And the derelict probably has bacteria or microbes to help it function while it's alive. When it dies, they may well eat the ship. Between the planet and the ship's chemistry, it's likely the ship ages more quickly than we think. After a time, its possible the entire thing would collapse. Maybe no retcon.
I agree with this. In terms of RETCONs this is explainable to me. Especially concerning what has been said in ALIEN regarding the DERELICT, people have ignored all kinds of details in assuming the crew of the NOSTROMO just didn't know what they were seeing in there. For example: Kane clearly states the Egg Silo is a 'CAVE' but almost no one considers it one.
In the case of Alien chemistry interacting with metal, I mean, that isn't understandable whatsoever, so why should it be strange to say 'The climate on the planet in addition to the chemical changes with the Goo are why the Derelict appears fossilized?
Also, I already got the impression that they said FOSSILIZED, BECAUSE they can't understand its an exoskeleton. Right? In which case, that has already been retcon-ed or explained away by misunderstanding, based on human error, and making observations before they encountered the living version of the alien.