The giant parking lots and 6 to 8 lane roads are what would need to go in order to make a place walkable on the level of SF or NYC. Additionally, a lot of the smaller, quirky high density living in these cities is because they didn’t have to comply with ADA at the time of construction. If you start adding elevators and ramps and the space needed in passages and doorways for wheelchairs to turn around, you will end up with a much larger footprint.

Granted, ADA and other upgrades in quality of life are nowhere near the big problem of the cost of reducing lot and block sizes retroactively. The design for cities made for cars and for pedestrians are fundamentally opposed, and cannot coexist.

Moving away from car-friendliness will do so much more for density and walk-ability than doing away with ADA compliance, by easily a factor of 1000. I built a house which is largely ADA compliant, and lost maybe 10 square feet, most of that was due to converting traditional doors to pocket doors, which requires wider walls. For the most part, all of the doorways could accommodate the wider door requirements without moving any walls.

If I got rid of my oversized two-car garage, I could recover maybe 400 square feet. Without a driveway, another house of the same exact size could be placed on the same lot.

Cars are antithetical to density. Discouraging their use will greatly improve city density, while also having environmental benefits.

I don't have a citation for you, but it sure seems that something requires new construction apartments in SF to devote half of their square footage to wheelchair accessible bathrooms... I'd guess every apartment I lived in in SF would be illegal to construct today due to things like clearances.