The thing is, if I'm just referring to an unspecified hash in a cipher I'm proposing, it's coming with the implicit assumption that (a) the hash is sufficiently

The thing is, if I'm just referring to an unspecified hash in a cipher I'm proposing, it's coming with the implicit assumption that (a) the hash is sufficiently good to make the scheme work, and (b) such a hash is actually available. To me, in a discussion like this, "your scheme sucks because SHA2 would break it" is fundamentally the same as "your scheme sucks because CRC would break it": from a mathematical standpoint it's just nonsensical (you can always find a bad hash function to break a cipher), and from a practical standpoint, while I agree it's a "big deal" (e.g., if I were implementing such a cipher, I would surely put up guardrails to prevent people using such a hash), it's also the least interesting way to shoot down the cipher I'm trying to discuss.