> Cologne is a city with roots that reach back to the Roman empire. It has grown and developed over hundreds of years. Where would all these underground parking lots come from?

If car parking is a worthwhile use of space, the market will provide it (and whether below or above ground is beside the point). If it's not, it won't.

> Why not go a step further and demand that all streets should be put underground, too, so that pedestrians can enjoy an idle stroll on green meadors above ground without all the annoyances that come from car traffic?

It's a good idea, but we probably have to get there incrementally.