This is basically been the case for decades in the U.S.
Other countries seem to get by just fine with living in cities and having kids.
Other countries don't concentrate all their disadvantaged people in the cities.
With that said, it's perhaps possible that millennial pushing for changes and upgrades and improvements and the fundamental human right of quality education for all might not yield an ideal outcome. Improving schools can very reasonably be expected to be multi-decade project of changing budgets, priorities, staffing, training, expectations, culture, and more. Someone with a toddler might want quality schools in the next three years instead of next thirty. This might, for some, incentivize a different set of somewhat more selfish reactions.
You're completely right. Millenials can, should, and are morally obligated to push for improvements and upgrades. But there may be a wrinkle or three.
With that said, is it possible that high-potential children could be viewed as something other than interchangeable widgets whose primary purpose is to make public schools that are failing many students look good? Especially given that such schools are often little better at meeting the needs of the high-potential children than they are all of the other children.
You're right. Charter schools are often guilty of creaming. Yet, are we doing anyone a favor by sacrificing the education of high-potential children to mask problems?