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
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).
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.