Diceware proper is not 'plucked from actual phrases that have been uttered'. It's a random sequence obtained by throwing a dice a number of times (hence the name). This means you can't lookup your e-book collection for an exact phrase to use (eg. a famous quote).

Moreover, diceware can be set up with any dictionary, and nobody stops you from computing your own dictionary. If you do that, good luck to the bruter attempting to break your passphrase!

BTW there is no limit on the number of terms in your diceware passphrase, so if/when brute force makes 5-term phrases too weak, users may just add one or two more terms.