As I shared in another comment which I think is relevant to your points, the biggest losers were Los Angeles County, California (-90,704); Cook County, Illinois (-68,314); Queens County, New York (-50,112); Kings County, New York (-46,970); and Bronx County, New York (-41,143).

I think all of the "legacy" cities posting losses larger than Philly (though we're obviously in this category too) does point to a larger trend like you said.

Even major "Sun Belt" cities like Miami, Houston and Dallas, all had sizable domestic out-migration: Miami-Dade County, FL (-38,203); Harris County, TX (-20,006); and Dallas County, TX (-20,245). All of this is assuming this data is remotely accurate.

Gen Z is still moving to cities and it's encouraging that Philly is one of their top destinations. We had a higher net gain than NYC, Chicago, Houston, Charlotte, etc. I wasn't alive in the 70s but wasn't that the immediate aftermath of white flight? I don't think things will get anywhere near that bad.