I don't go downtown very often, so I don't have much of an emotional investment in what happens. I think that compared to the way the downtown malls were, anything would be an improvement.
I don't think a sky bridge would be the death of main street. It just doesn't seem like that big of a deal to me. The old malls forced everyone out onto main street to go from one to the other, and that didn't seem to help "keep main street alive".
Todays article in the Salt Lake Tribune about Rocky's Veto on the sky bridge mentioned Rocky's idea of closing main street to cars. Trax would still go through, obviously, but I think the idea bears some contemplation. The ZCMI center food court always had people in it, eating and talking (at least during my teen-age years when I was at the mall more often). Imagine if that type of atmosphere could be transferred outside. If main street was closed to traffic, and an inviting atmosphere was created to attract people, it wouldn't matter if there was a sky bridge or not, because people would want to be on main street.
I think the key here is attracting people to main street instead of trying to force them to pass through it (by doing away with the sky bridge idea) on their way to the other mall.