Well, I have read some discussions, and it seems they started degraining their releases after the first JP bluray of Nausicaä. There are some online pics of the original JP bluray of Nausicaä, and in those pics you can see that, despite the faded colors (although most of the red comes from the different color temperature they use in Japan), it is quite grainy (as it should be), whereas the second JP bluray of Nausicaä is much colder and the grain structure is not well defined.
There may be some additional problems I’m unaware of, though.
I would donate to fan projects involving scanning original japanese prints. It would be interesting to see how it looked back then in theaters.