It was that old Law of (Unintended?) Consequences: What Democrat voters and political leaders refuse to believe
Well, they do believe it but only Moynihan was willing to say it out loud. The real goal was achieved: Generations of Dem voters hooked on dependency, uselessness, and hopelessness. Of course, that applies not only to African-Americans. There are far more white than black people in that category.
However, there were no dysfunctional black urban ghettos before Johnson's Great Society eliminated poverty and hopelessness.