Holland (a province in The Netherlands) is relatively flat, The Netherlands is not. If you ride the route for races such as Amstel Gold you'll find out. And yet people still ride upright Dutch bikes in the hilly areas.

More so, these are the preferred bikes for people all over the world. They're ubiquitous in Sweden, Norway, Finland, Germany, Belgium, Switzerland, Austria, Italy, Spain, France, China, and elsewhere. In the late 90's there was a move towards other styles but that lasted only about seven years and shops say that demand went back to Dutch uprights. That's what people who use their bicycles for daily transportation prefer. Some people with very long commutes (greater than maybe 15 miles) or lots of steep hills will ride hybrid or road bikes but even for longer commutes you still see far more uprights than anything.

This excuse that we can't use them here because we have hills is a fallacy.

This also ignores that even in the hilliest places the majority of shorter distance errands for eating, shopping, school, or groceries are relatively flat and that it is only for the longer distance trips that many hills are encountered. Ride the bike for the short local stuff and drive a car for the fewer trips that involve mountains.