Have we ruled out simple substitution unwisely?

[MarcoP]

Yes, one can cherry pick a few similar things for certain languages, but in Voynichese they are systematic (hence the low entropy). Once a certain Jorge Stolfi even wrote a simple grammar that defines the constrained nature of most Voynichese words. I don't know about mandarin, but you cannot do that for European languages.

Also, you had to cherry pick bigrams instead of characters... this tells a lot