I think you will have considerable difficulty in persuading either lawyers or anyone else that "unjust" does not mean "contrary to justice". Neither are they likely to accept that controversies over what is "unjust" should lead to abandonment of that sense of the word.

As to whether this ordinary (and almost universal) sense of the word is "subjective", I would point out that there is considerable controversy about the scope of "unjust enrichment", the role of "injustice" within it, and the ways in which "injustice" should be described and analyzed. So it is far from obvious that your preferred meaning is any less subjective.

Steve Hedley
Faculty of Law, University College, Cork