It’s an interrogative that gets right at the undercurrent of social commentary that runs through the 1974 series, and reminds viewers that man, after a fashion, is responsible -- in this universe at least -- for his own destruction.
So why does the city look so...mid-20th century?
To put a fine point on the matter: Destruction is humanity's legacy.
In part, this is because Virdon is an optimist. Even after everything, he would rather see the good in mankind than the evil.
Eventually, this human family comes together, and escapes the trap Zaius has set. Perhaps our legacy is that in bad times, we stick together, and accept others into our "tribe."