The answer is to rework city zoning laws to allow for more vertical building and less car parking requirements, but $$$, nimbyism and "I was here first" thinking gets in the way. Way more people want to live in cities than can afford it and it's a shame that cities aren't thinking about this enough.

It's a people problem, not a technology one - the elevator was already invented.

"The Rent Is Too Damn High" is a good, short book on this.