We're clearly talking past each other. You seem to think that I'm saying that the stronger the seed is, the stronger the generated keys will be. That's obviously wrong and it's not what I'm saying.

What I'm saying is that it doesn't make sense to choose a seed strength based on the security level of private keys. It makes sense to say "128 bits is enough security for my needs, that's why I use 12-word seeds". It doesn't make sense to say "I use 12-word seeds because private keys have 128 bits of security". You see what I'm saying?

The two things have different jobs. The security level of a private key protects a single satoshi balance. The security level of a seed protects every private key in your wallet. The idea that the seed can't make use of more than 128 bits of security is wrong.

No it's not. It's an excellent example that exposes the flawed reasoning I'm talking about perfectly.