North American cities were fairly dense before World War II. After the war ended, it was the age of automobile and suburbs which led to the hollowing out of cities and many buildings were demolished and turned into parking lots.
Now with people coming back into cities, cities are becoming dense again and many parking lots are now being re-developed.
Toronto has gone under a massive development since 1990's and the sea of parking lots are almost gone. By 2030, I think most of Toronto's remaining under utilized areas will have been developed.
The yellowbelt will undoubtedly remain by 2036.