Do you value the various Chinatowns in cities like New York and San Francisco. To me, these unplanned, spontaneous developments is what makes a city worthwhile. Modern urban planners seem to want to make such spontaneity impossible, and will end up as sterile and uninviting as a strip mall.
We could have empty places interspersed in the city in which people living in the city could use as their spontaneity desires. It would also leave some room for upgrading etc
But these places arise precisely from the fact that they're not empty. Most chinatowns/koreatowns are in places where, back when the immigration wave was happenning, there was cheap housing that was close enough to local commerce. As soon as one or two immigrants went there, friends and family followed, and soon there was a powerful support network for immigrants, and the community thrived. You can't build that in an empty space.
And upgrading is handled well enough today by (a) knocking down old buildings and build new, bigger ones (except this doesn't sit well with planning) and (b) expanding the city to a different direction that can be developed according to market values and needs, which also undercuts planning (since these new areas won't at first have the infrastructure of an urban center).
A city, by traditional definition, doesn't contain empty spaces. The outside of a city is where the empty space lies.
Empty spaces degrade the city because it takes time to walk through an empty space just to get somewhere. And a city is meant to be walked around.
A traditional piazza or a square is fine because they occupy one city block, blocks are traditionally small, and they're surrounded by dense array of buildings: shops and apartments.
