A quick Google search suggests English has about 10 bits of entropy per word. Having a long password like that can still have high total entropy I suppose, but it has a low entropy density.
Maybe 10 bits is the average over the dictionary – which is what matters here, but over normal text it is significantly less. Our best current estimation for relatively high-level text (texts published by the EU) is 6 bits per word[1].
However, as our methods of predicting text improve, this number is revised down. LLMs ought to have made a serious dent in it, but I haven't looked up any newer results.
Anyway, all of this to say is that which words are chosen matters, but how they are put together matters perhaps more.
The diceware method is supposed to generate totally random words, so it should fundamentally be unpredictable unless there's a flaw in the source of randomness.
