ehhh...how to say this politely...You're wrong.
First of all, its 1626 words. 1626^12 = 3.4 * 10^38.
It's the same as 2^128, or 128 bits of security, which is the same for any bitcoin address
that has already spent funds. ECDSA for Bitcoin is 256 bits which provides 128 bits of
security. Unspent addresses benefit from additional security because of RIPEMD-160 hash,
increasing it to 160 bits. However, electrum uses key stretching of an 100,000 round hash
once you know the seed, increasing the security to 144 bits.
You ARE correct that human generated phrases POTENTIALLY could have much higher
levels of security, but it doesn't matter because A) 128 bits is beyond brute forcing
by any form of classical computing (do the math and see how many super computers
and millions of years you need) and B) Bitcoin is limited to 160 bits of security no
matter what.
However, its impossible to measure with certainty the entropy level of a human generated
phrase and potential for error exists that does not exist with computer generated pass phrases.
Great explanation, thank you!
And it's not rude to say that someone is just wrong, we can't know everything
However, as the two methods give presumably good enough protection, we are still on the line that a human generated phrase can be remembered more easily than a group of random words.
It wouldn't require too much work at all to allow a user to write his sentence, check that he uses caps, lower letters, punctuation and possibly numbers.
The random words method could be left active as an option.
