Prior to 2007, gifted and talented classes were skewed towards white and asians and the admissions process was criticized for allowing "favoritism and discrimination"[1]. A standardized test was then implemented for 4 year olds going forward, but over subsequent years people realized that this resulted in even lower hispanic and black enrollment into the coveted G&T programs[2].
de Blasio is pushing for a more ethnically diverse mix across all schools. Based on the K G&T admissions changes made in 2007, he is fairly sure this change will achieve his objective. He firmly subscribes to the idea that while certain elite schools will be by definition diluted in terms of academic achievement and excellence, the overall NYC school system will benefit. He fervently believes that trying to increase the global median achievement at the expense of a lower academic peak achieved by a tiny minority of students is worth it (aka YCMAOWBE[3] or TANSTAAFL[4]).
While I agree with the sentiment that one should try to fix the entire pipeline and not focus on the end product (too little too late), critics lambasting de Blasio's plan need to understand that he's simply making different trade-offs during his last term in office as mayor.
[1] https://www.nytimes.com/2006/09/26/nyregion/26gifted.html
[2] https://hechingerreport.org/as-new-york-struggles-to-make-gi...
[3] https://en.wiktionary.org/wiki/you_can%27t_make_an_omelette_...
[4] https://en.wikipedia.org/wiki/There_ain%27t_no_such_thing_as...