> I've heard urban planners (in lectures) say that when there is not a row of parked cars between a street and the sidewalk, people are much less likely to walk and spend time there.

Have they considered replacing the parking lane with a bike lane instead of regular traffic?

> And, as others pointed out, in most cities it is unrealistic due to how little parking there is available.

That's purely a supply and demand issue--because of how cheap street parking is, the price of parking is very low and thus elevated parking garages are uneconomical. If only elevated parking is available, that changes.