An interesting question.
The first two phrases you give as examples ('Boys will be boys' and 'War is war') are such frequently used expressions that they now have a fixed meaning - that boys behave badly and that bad things happen in war time.
The use of other phrases, such as 'Phrasal verbs will be phrasal verbs', for example, although not commonly used, would imply the same attitude of resigned acceptance of the negative aspects of the subject of the sentence. Maybe the implication is 'It's just the way things are, so there's not much we can do about it.'
Where the structure breaks down is much harder to define. I suppose, when a phrase is too long and too cumbersome, or when a phrase implies too much knowledge to refer to something everyone accepts as a truth, the structure no longer works as well. For example, you aren't going to hear many people making sentences like 'Ah well, Post-Hegelian dialecticism will be Post-Hegelian dialecticism.' - or if you did, then not many listeners would get anything meaningful out of the utterance.
Maybe an academic linguistics site though a university might have a forum. I don't know.
If you find one, it would be interesting to hear about it.
- For Teachers