"Assuming there was no just cause" is a huge assumption. I will make a few other huge assumptions–I assume we are speaking of husband and wife, (because otherwise the discussion is really about marital relations outside of marriage) and I’ll assume that they are chaste during the woman’s fertile times, (otherwise it becomes a discussion about those acts) and I’ll assume that they would not turn to abortion should the wife become pregnant, (because then the argument becomes about abortion.) If all of those assumptions held true, then if a husband and wife desire to use NFP to postpone pregnancy, then I would not try to argue with them at all. The nature of using NFP effectively requires the couple to practice self-sacrifice, and I would expect that they would either eventually change their minds and welcome a child/children or they would learn and achieve a heroic level of self-sacrifice for our present society.
It would seem highly likely to me that at some point one of the spouses would change his/her mind about wanting a child/children while the other didn’t, and that might lead to marital troubles and potentially divorce.
Otherwise, my concern/arguments against it would be more related to those big assumption I stated earlier that enter into other discussions about chastity< (both within and outside of marriage) and abortion. (And I’ll also mention regarding the op first assumption that if two people who are married to each other really never, ever want children, then I’d have to think that somewhere underlying that is potentially some type of “just” cause, since the Church has not yet ever definitively stated what is or isn’t a just cause.)