We have already figured out how to build beautiful buildings in any number of styles from many eras. That's because a building is a discrete entity whose design is managed by one group of people, who can form a relationship with an architect who pours their heart and soul into the design.
A street network, on the other hand, is managed by successive governments staffed by much more disinterested operators, and often by different governments who may be working at cross purposes. I'm not saying there should be a master plan (I actually don't like those) but rather that the logistics of designing a street network are much, much harder than for a single building, however you go about it.
I also consider the harmonization of automobile and pedestrian traffic to be an open problem. Maximalists on both sides just talk past each other. Car-oriented development is bad, but at the same time we aren't going back to a pre-car world.
Whether your model is Brasilia (modern) or Cayala (traditional), there are just so few opportunities to actually think about the design of the street network of a city. Street networks are just taken as given and then architects try to make the best of them. That's not a criticism of architects or city planners; the street network design largely seems to be on autopilot with very few opportunities for a change in direction. And for established cities, changing the network is nearly impossible.