We need buildings to live in, but
not the ones we have. To create places where people can walk or ride to everything they need in their daily life, many existing structures will need to be re-purposed. And it turns out that it's
very hard to turn commercial into residential.
https://99percentinvisible.org/episode/office-space/
And that's just the building part. You also have to deal with the fact that cities are already overstressed, creating ecological disasters as soon as you go outside of the privileged core. Don't look at walkable Manhattan or Brooklyn without also looking at uber-ugly Bronx or Newark, which have to exist to serve those central cores. Or the dumps all over New York state. Or the toxic plumes in the air and water. The transit system, the water and sewage, trash removal, etc. would all creak under the strain of even more population, not to mention the problem of increased demand making housing even more unaffordable for all but the most privileged. Repeat for LA, Houston, Chicago, etc. No, "everyone move to the mega-city" is not the solution some think it is. The fact is that big cities aren't free of these problems; they just export them elsewhere.
It actually turns out that it would be a lot easier to convert residential in commercial than the other way around, in suburbs and small towns where they are. There's still lots of complexity, of course. You have to fix zoning. You have to make that "good infrastructure" work in many places instead of a few. You have to convince people to pay for it, and yes, there will still be a lot of tearing down and rebuilding with all of the environmental costs that entails. As GP mentioned. It's not easy by any means, but creating less car-centric environments this way is more feasible than mass migration to cities and more sustainable too. Some day cars will not be necessary for the majority of the population, I really do hope we get there in my lifetime, but that day is not today and getting there will take orders of magnitude more effort than the "abolish cars now" crowd seems capable of imagining.
Magic-wand thinking doesn't get us anywhere. Practical, incremental improvements do. And reducing particulates from tires is one of those.