Hi Dimi,
It is certainly not convincing as we know (at least) one member of the Belgian parliament who proposed this law change to implement Freedom of Panorama.
Second, there were ammendments from other parties in the Belgian parliament to make it only non-commercial, but those did not get a majority in parliament.
Also this law change was done to make it legally possible for people to share photos of (modern) buildings on social media and Wikipedia, both requiring commercial usage to be allowed.
As you indicated, the normal exploitation of a design from an architect is to sell the design and get the building built, and not to do anything with the photos taken in the public environment of the building.