Here's an example of the required infrastructure: https://fitzrovianews.files.wordpress.com/2013/03/warren-st-...

Seems viable though, as some cities are moving in that direction. Barcelona for example, implemented "superblocks" where traffic is closed off in the middle https://www.barcelona.cat/infobarcelona/en/over-8000-square-...

Closing streets can actually improve traffic. It many cities, you could close many side-streets to all but local traffic (residents). This would make the city much more bike & scooter friendly.

Electric bikes and scooters really change the game, you no longer have to be in great shape or sweaty in order to avoid taking a car to your destination. Really, safety is the main concern many people have.