Slums constructed to hide the poor are not the same thing developers trying to sell units would choose to build. Initially developers would target the high-end (this is what they do now with supply limits in place), then when demand in the high-end is exhausted they would build for the middle class. When the middle class upgrades into newer units, older homes they leave behind will become available to those lower on the ladder.

This can be supplemented to benefit the poor sooner by eg. requiring a subset of units in high-end developments to be subsidized for people with lower incomes.