The article hints that Berlin's soil might have been unsuitable for some of the more monstrously monumental buildings conceived, and in fact, one of the surviving pieces of Speer's plans is the Schwerbelastungskörper, a massive block of concrete built to study just how much such a building would sink over time: https://en.wikipedia.org/wiki/Schwerbelastungskörper

After the war, it first seemed too massive to demolish with reasonable effort, and now it is protected as some kind of ozymandian monument.