We are obviously way off topic in this thread (mods maybe break into a new thread?), but I see at least three major problems with the "no cars/many fewer cars" theory:
1) It assumes that the trend of moving back to urban areas will continue. I don't necessarily see that as a given - historically we have seen something of an ebb and flow between cities and suburbs and I could see the same happening in the future. At a certain point cities are going to become even more crowded than they already are, crime may pick back up, property values will rise, etc.
2) Even if people do move to cities, the theory assumes that the cities they are moving to will have widespread, affordable alternative transit options - people aren't going to be willing to take Ubers/Lyfts to work and back every day. Put another way - it makes sense that people in Manhattan might not need cars, but how many people living in LA don't have cars now (even if they also occasionally take Ubers/Lyfts to bars, etc.)?
3) Finally, it assumes that as Millennials continue to age and actually start families they will be content to continue using public transportation or Ubers/Lyfts, which again I'm not sure is true. To use a personal example: my girlfriend has many relatives in the Westchester area of upstate New York, and currently we don't have much of an issue taking the subway to the MetroNorth, then taking an Uber from the MetroNorth to her relatives' house. But if you add two young kids into the mix that trip would be CONSIDERABLY more costly and annoying in the absence of a privately-owned car that would just take us directly from A-B.