Sustainability and efficiency, sure - you're definitely right on that. I'm going to take a bit of a devil's advocate role here though:
There are negative impacts to dense packing of humans too, though. Think about the local ecosystem of plants and animals that was irreparably destroyed and will never be recovered in the construction of X densely packed city you can think of. Think about the huge scale of resource shifting in the geographic region (water, food, electricity generation) that has to occur in the surrounding area which negatively impacts not only the city but the environments it pulls those resources out of.
Sprawl leaves room to interweave humans with the rest of the natural world in a way in which densely packed cities do not. It leaves room for trees to grow, critters to roam, rain water to be reclaimed into aquifers. It also spreads the strain of resource extraction and reduces the impact from hot spots at the most granular level.
Car focussed development destroys far more land (e.g. parking) than similar size developments that enable walking/cycling/public transport.
Look at London - most people don't bother driving into the center of London andit's technically counted as a forest due to all the greenery. When you design for cars, all other travel modes are made impractical as cars take up so much room that all the facilities end up being miles away from people.
??? Sprawl is actually environmentally friendly? What in the world? Densely packed cities by definition take up less space than suburbs.