If the intention of a criticism is not to construct then call it what it is: a denunciation. That would save us all a lot of time.
I would not denounce denunciation – sometimes there are things worthy. However, it is not an act that should be taken lightly. Nor should it be confused with criticism, which is an opening, not a closing.
To denounce is to decide (or to claim to). It is to cut off, to load up the proverbial iceberg and push it out to sea.
If 'constructive criticism' were a pleonasm then critique might be less deserving of its critics (not to mention its denunciators).