The Social Justice movement has two kinds of people. The thought leaders in the universities are functionally revolutionaires: "Justice" doesn't mean fairness in their thinking, it means "privileging" the oppressed group and de-privileging the oppressor group. (See the book "Cynical Theories" for a thorough--and left-leaning--analysis of the development of the academic ideas from disillusioned Marxists Foucault/Derrier to the current state.) This isn't really "justice" in the sense of fairness, it is retribution: attempting to right a wrong by wronging.

Then there are the masses, who think "Justice" means "justice" and more or less buy the narrative. (At least, as far as I can tell.) These days, activism is how a lot of people seem to derive their meaning in life. So, "defund the police" advances the revolutionary cause, while being a pithy cause activists can rally around if they are not of the critical thinking persuasion.

The lack of thought about the after-effects is from the academic thinking. The goal is simply to tear down the oppressive system; there is no thought given to a replacement system or how that system would avoid simply being a different oppressing system. Or whether a replacement system would actually be less oppressive than the current system, which by historical standards is one of the least oppressive.