I think that if enough people were carrying concealed, and if that fact were common knowledge, then there would in fact be an "umbrella" effect. What percentage of the population would have to be carrying to make the average thug actually think about this as a real possibility is probably a lot higher than any place in the U.S. currently experiences. There are about 16 million licensed concealed carriers in the country, and even if you assume that most of them carry all the time (a big assumption) and that there are several million more carrying who are in states where no license is required, we still probably have no more than 20 million or so carrying out of 330 million people. I doubt that this small percentage is enough to impinge on the thought process of lowlife thugs intent on mugging and robbing victims.
About 55 years ago I had a part time job as a waiter in a catering hall. One night we hosted a retirement dinner for a police captain, and the sign posted outside the catering hall announced what the event was and who was being honored. In those days it was required for all off duty police to be carrying, and everyone knew that. I have no doubt that no thug would have even considered trying to rob anyone at that event, knowing virtually everyone present was carrying a loaded firearm (in those days, likely a snubbie 38). But short of that kind of situation, I don't think there is much "umbrella" effect, and certainly not on a mostly deserted street at 1 AM.