Hmmm...I think opinion is mixed on skywalks. They're great in the winter, especially in places like Minneapolis, and Calgary (which calls them "Plus 15's").
Toronto and Montreal have gone the other way, and developed extensive networks of underground passages connecting office buildings, shopping malls, and subway/commuter rail stations. I don't know about Minneapolis or Calgary, but in Toronto and Montreal I don't think street life has suffered. In fact, in Toronto's case, if it weren't for the underground walkways, it would be very difficult to manage conflicts between pedestrians, automobiles, and surface transit - although conversely, perhaps we would be able to justify the development of pedestrian streets/malls if the underground system was not in place.