Nice figurative use of the word descent there.
Would you also object to "train wreck" used metaphorically being catalogued in the dictionary? Are these not useful phrases for a language learner? How about "danger money"? How about "white lie"? "Red line"? Where is your red line when it comes to these things?
Anyway maybe dictionaries don't only enumerate bare words. Who made that rule? Lexicographically why make a distinction between single word nouns and multi-word noun phrases? Also, would you ban phrasal verbs. We're allowed to enumerate "set" but "set down", "set up", "set upon", "set aside", … all these are verboten or are they A-OK?