Why is a known-plaintext attack considered a complete break?
if a cipher has a known-plaintext attack, then it is considered completely broken.


Yes, pretty much...


  [Paraphrased] But can't we come up with a case where this isn't true, such as a One Time Pad?


Yes, we can come up with cases like that; however the requirements of such a case (key as long as the plaintext, no key reuse) make such a cipher impractical.

Instead, when we talk about such a cipher:


We generally use the same key to encrypt multiple messages (for example, consider TLS, where the the symmetric key is used to encrypt all records going in the same direction)
And, even when we use a key to encrypt only one message, the attacker might be able to guess parts of it; if he can use those guessed parts as known plaintexts, and recover the parts he didn't know (or even to verify if his guess is accurate), well, the cipher didn't do its job.


"Known plaintexts implies total break" is an excellent rule-of-thumb, even if you can come up with cases where it is not true.