A lot of the good road bike trails in the US are built next to slow-moving rivers and canals for that very reason. Rivers tend to carve the land or reroute until the slope is gentle and nearly constant. My favorite way to find road bike trails in the US is by looking near large rivers.

For bikes replacing cars, you need flat paths to places you want to go (your home, shopping, work) not to places you go to ride a bike.

That is nice if you're riding bike on trails you can pick, for fun (thanks for hint).

For those of us commuting to work, to move from place A to place B and back, that is unfortunately not always feasible...

The wind there is much worse than small hills!

No it's not. As a Dutch person who moved to Switzerland I can say that it's much easier to bike against the wind than to bike up-hill.

Down south (Limburg etc) is a lot more hilly (like a LOT); how is the cycling culture there? I've only been there on vacation once :p.

There is a lot of cycling in Limburg as well. Especially sport cycling ("wielrennen"), not despite but rather because of the hills!

I grew up there and my friends and I used our (regular) bikes a lot to get from town to town. Because of the hills it was exhausting but doable. When I went to study up north, I remember being surprised how easy cycling is in the rest of our (flat) country: you can easily go on for hours!