You make cities affordable by unrestricting building height way beyond the "projected" demand for residential construction in the core. Then as dense, cheap res

You make cities affordable by unrestricting building height way beyond the "projected" demand for residential construction in the core. Then as dense, cheap residential fills up you will see the natural build out of more of it. Keeping space to grow / build public transit and infrastructure is also critical. That doesn't mean overbuild now, it means don't try to stop it when the time comes.