First off, cities are a reasonably efficient way for a large number of people to live together. What about the countryside? Well apart from those who are self-sufficient on it, they need to export their labour and import resources in low numbers. The environmental impact is probably lower in the countryside if development is sympathetic (which often it is not).
Cities also provide for the social aspects of life. And Covid lockdown has not replaced the need for entertainment on a large scale, such as football and theatre, cinema and restaurants. With effort the transport for these events is relatively simple and environmentally friendly in the city.
What cities need is a comprehensive environmental plan which considers economics rather than the other way round. Soviet style tower blocks, and kennel like suburbs must all be banished. City centres need walls of plants and roof gardens, suburbs need communal areas. Cities need lungs and spaces that allow creativity. Not everyone wants their own garden, but all will enjoy the use of a communal space. These spaces do not need to be huge, but I would suggest that they should be multi-generational where possible. Have we created a false separation of age groups? I remember the Red Lion in Newlyn where the younger ones went to the top bar with a juke box, and the older ones in the bottom bar, they were L shaped with the toilets at the back and the door at the front. Separate but together, and no tension. With covid maybe more street cafes and open shared spaces. Sports grounds and facilities should be more multi-sex.
many jobs will be more home based than before, thus less commuting and less business for the city centre. Maybe we need to move away from the business rates model that concentrates activities in a centrifuge? The 15 minute city is an aspiration, or could be if overlaid with a 60 minute city. Not everything can be provided in a 15 minute walk radius.
Then we come to the interaction between cities and the countryside / rest of the world. Power and goods in, waste out. But the relationship between the urban and rural needs to be an equable one. The countryside is not just a rubbish dump for the city, a place for power stations and airport, intensive farms and intensive style leisure facilities. Both should be learning environments for our children.