As much as I love to support biking it's important to keep in mind how geography dependent it can be. Denmark and The Netherlands are extremely flat, which makes them unusually suitable locations for biking. If you add in any significant amount of elevation change then you immediately raise the bar in terms of the fitness requirements necessary to get to typical destinations.
I don't think most cities have so much elevation gain that biking to work isn't practical for most people. There are definitely exceptions, but I'm pretty sure a person of average fitness could bike to work without too much trouble in the vast majority of cases.
The majority of bikes sold have multiple gears, so hills should be even less of an issue.
Electric assist bikes are making this far less of an issue. My relatively aged parents had no trouble riding 70km with 1800m of climbing. The happiness, health, and environmental benefits remain.
