Ta'anis 29a teaches that since by the Meraglim the Jews cried for naught, that night, which was Tisha B'av, would be established as a night of crying for all generations.
Why should this be? What did their offspring do, that they should be punished? Sure, HaShem visits the sin until the fourth generation (Shemos 20:5)1, but the Churban was much more than four generations later.
1Makkos 24a notes that this was one of four statements that Moshe said that later prophets overrode. However, at this point in time, it still remained in effect. Perhaps one can argue that at the time of Yechezkel, this statement should be overturned retroactively, which would strengthen this question.