reph said:
The explanation doesn't convince me. I don't hear an elided "are a" in "You bastard!"
What about name calling without the "you"? Someone addressing an opponent in an argument says "Jerk!" or "Creep!" (To forestall the jokes, I'll specify that these are not instructions.) It doesn't seem plausible that the one word is what remains of a ghostly sentence. Similarly, if the neighbor annoyed by youthful noise or vandalism says "Kids today!" or "Those kids!" or just "Kids!" I can't confidently expand that utterance to something that contains a verb and say that the one or two words that were spoken were a short form of the whole thing.
It is most definitely an elided sentence. If it weren't, you would not be able to interpret what was said to you.
English is a "wordy" language. As speakers, we will cut out the extra stuff to conserve words, time, breath, energy, etc.
The construction of "You Bastard!" or "Jerk!" is:
You{are a} Bastard!
You are aJerk!
The fact that all native speakers (outside of pun-humour pieces) agree to this means the words are there even if you don't hear it.
With "kids today" or "Those kids" you are dealing with idioms. (Like "The cats out of the bag")
When a person hears "Kids today". They are able to interpret it as:
Kids today{are so crazy and nothing like we were when we were young}
It is entirely plausible that these are ghosts of sentences because that is exactly what they are. As native speakers of a language we can agree on conventions in the spoken word that would not be acceptable in prescriptive grammar. One of these conventions is removing verb "to be" when paired with an indefinite noun. (Notice you cannot say "You
{are the} best.")
At any rate, in dialogue pretty much anything goes. No one really speaks in perfect grammatical sentences. Just be careful about these rules when writing prose...
Good Luck

10000