Let me spell it out for you, by giving you a local example that is both hypothetical and hyperbolic.
L’Booshondria used to live in a hovel in Shaw with her eight kids by seven different men, four of those men are currently in prison. But she got an Affirmatively Furthering Fair Housing voucher to move her and all her kids into the Canfield Green housing project in Ferguson, where she can teach her sons how to be gentle giants and swipe cigarillos. Once she moved out, domestic partners Adam and Steve buy the Shaw hovel and renovate it and live there themselves. This means that in the context of that one residential parcel, the population has decreased from nine to two. Lather rinse repeat, and it’s easy to see why the population of the City of St. Louis is both declining and the city’s overall condition is getting better. Adam and Steve living in that parcel means that the actual land and the house are worth a lot more, a lot more taxes are being collected from it, and also Adam and Steve are generating income and spending money which generates even more tax revenue, and on top of that, the cops aren’t being called on them every other day. The city earnings tax never applied to L’Booshondria’s crazy checks for all her kids, and the city’s sales tax on food never applied to the EBT card. Also, when L’Booshondria was living in the hovel, the hovel wasn’t worth that much, therefore, not much in the way of property taxes could be collected from it.
Otherwise known as addition by subtraction.
This is why you can’t use simple raw population numbers of urban areas, see a decline, and therefore conclude that gentrification isn’t happening. The number cruncher sees nine people before and two people now and figures that a disaster must have happened, because that’s all the number cruncher knows.
Other factors that this writer seem to miss are: Immigration (growth of sunbelt metros), whites moving out of older inner suburbs to move to newer outer suburbs and exurbs (getting away from their new AFFH voucher L’Booshondria neighbors), and that it makes a lot of sense to live in close proximity to an institution that spends $4 trillion a year (Washington D.C.’s true population growth bucking the national trend).