The known 2048 words at 128 = 2^128 and IF (and its a big if) there was a sufficently large qbit computuer, becomes 2^64 problem that is a lot more crackabble.
Why did you halve that?
On elliptic curve, a 256-bit private key provides half the security meaning 128 bits of security. But when you have an entropy for key derivation, that entropy is providing the same amount of security as its size. Meaning a 128 bit entropy provides 128 bits of security not 64.