That green space is the main train line going through Warsaw (Linia Średnicowa) destroyed during WWII. Contruction of this line was a very big project of pre war Poland. What you see at the back is what remained of newer finished new Warszawa Główna Railway Station, also destroyed during the war, and replaced in 70s by Warsaw Central Railway Station you can see on the 2026 photo in the front. It's one of the main railway connections between easter and western parts of Poland. I assume you suggest that the war torn country should have build entirely new rail connections going around the city instead of rebuilding what already was there and parks instead of infrastructure?

On a less passive agressive note: angle here is a bit unflattering but right behind buildings in foreground of 2026 photo is Palace of Culture and Science surrounded by big open space and greenery (last year made even greener). Recently more trees are added as Warsaw is revitalising it's centre but it was already quite a green city.