Are you sure about that? Elon Musk said it was for "upgraded packs" back in 2015:
I think at that time these were new cells being produced, and it was experimental "baby steps", and thus perhaps they were only going into the 90 packs, since there were no 100 packs at that time, and he said it was chemistry for the "upgraded packs".
I hate to throw fuel on the fire (since I don't even really see this as a fire) but I'm not convinced that Tesla stuck with silicone once it appeared the 90 packs were degrading faster than the old 85's. Also,
@JonMc from Tesla told us it affects less than 1% of customers so it seems pretty limited in scope. But this is all just pure speculation on my part.
I miss the old days when we could discuss interesting things like this here without dramatic replies equating to Tesla is the devil, or conversely, is a saint, since Tesla is neither. It's just a corporation that is run by humans -- so it's flawed, like the rest of us.