Days before I moved to SF, I sold my car. I thought once about it, looked at the cost to park it, then sold it. Had parking in the city been substantially cheaper, I would have thought a couple of dozen times about selling the thing, and may very well have ended up keeping it, despite the fact that I would have almost never used it.
There are several 2D (ie. just-a-layer-of-blacktop, rather than a multi-story garage) parking lots in my area of the city. It's obvious that they would make more money as non-rent-controlled housing. It's somewhat clear that regulations in the city are keeping those lots open, and very clear that city regulations caused me to sell my car and use public transportation. :)
I'm pretty sure we'll be able to get away with fewer parking spaces in the long term, just like we built them guadually over the decades. But if we suddenly start eliminating them, traffic will be hell for the next 10-20 years.
Every time someone drives around a block to find somewhere to park, their car is unnecessarily burning fuel and spewing exhaust into the air. That means more pollution, more wasted time, lower productivity, and more accidents on residential streets because all those drivers looking for somewhere to park are not paying enough attention to what's in front of them.
This looks like just another of those naive proposals that assume that people will suddenly change their lifestyles given the right regulations. That's not realistic, especially when we're talking about car ownership in the United States of Automobiles.
What we need is actual data on car ownership trends, and a model to translate that data into the number of parking spaces that will be needed in a given city over the next couple of decades. Battery-powered and self-driving cars should also feature somewhere in that calculation. Only then can we ask how many parking spaces we should have.