@V13 You make an interesting point, which I will attempt to address.
If I have understood your comment correctly, you are in essence asking whether it is possible to make a non-random passphrase that is easier to memorize than a random passphrase, yet have equal or greater entropy. And if the answer is “yes”, then you are arguing that all else being equal (specifically — the entropy being equal) the passphrase that is easier to memorize will be more secure (because there is less risk that the user will engage in insecure practices such as writing down their master password, or choosing a password that is too short).
My response to that question is that, yes, it is possible that one could create a memorable, non-random passphrase that has sufficient strength (comparable to what can be achieved using randomly generated passphrases). However, the problem is that you can never be sure of the actual security/strength of such a non-random passphrase, because its entropy is unknowable.
Nonetheless, we can make some guesstimates. It has been claimed that the 3000 most common English words make up 90% of conversational English. To be conservative, let’s assume that your non-random passphrase consists only of words from among the top 1000 (or that the geometric mean of the words ranks is 1000). If the words were randomly chosen from this pool, you would get 10 bits of entropy per word. Now, research has shown that when words are arranged into grammatically correct language, the effective entropy is reduced by a factor of one half — thus, we would end up with 5 bits of entropy per word in a sentence. The average sentence contains around 14 words, and studies have shown that sentences longer than 17 words are difficult to read (and thus presumably difficult to memorize). So if we restrict ourselves to a sentence containing 16 words, the entropy can be estimated to be 16×5 = 80 bits.
Compared to a randomly generated passphrase that is drawn from a Diceware-style list of 7776 entries, the strength of the hypothetical 80-bit sentence is comparable to a 6-word passphrase that has a special character as a word separator (assuming the separator is randomly selected from a pool of 5-6 special characters, e.g.,
Of course, we can never be sure of the actual entropy of the non-random passphrase, because it is impossible to verify the many assumptions that go into estimating this value. And there is always a risk that an individual using this approach will choose a sentence that is a quote from a published work, in which case the entropy drops precipitously.