Sadly, I think this is only easy to accomplish in mid-sized densely-populated cities (<= 250000 inhabitants), unless there's a lot of planning and investments done.
Paradoxically, shopping associations were initially strongly opposed to implementing car-less downtown plans. Only to reckon later how wrong they were.
If I recall correctly, car-less policies were imported from Aarhus and Aalborg, where majors were experimenting with turning some main streets into pedestrian only by the late 1980s.
It's also interesting to note that both Aalborg and Oviedo tend to rank very highly in a few quality of life EU metrics [1]. I know both, and they are pretty fantastic places to work and live in. Everything is within short walking distance.
[1] http://ec.europa.eu/regional_policy/sources/docgener/studies...
You are right about the "easy" part, but it can definitely be done. In fact that's what Tokyo feels like, and it's the largest city/metro area in the world, with 38 million residents.
Many American and British cities fail in this regard due to excessively sparse housing. But, for example, I found downtown DC & Arlington very walkable and nice.
It's like being in the airport concourse with those electric golf cars racing between the pedestrians with their interminable annoying beeping.
If people don't have cars, parking won't be a problem, and walking from one store to another rather than paying for a taxi would become preferable. Then more people would have the awful experience of walking half a mile through a giant paved parking lot which would motivate people to change it.