The relevant variable here isn't the headline 52%, but the fraction of young people who lived independently but have recently stopped, which is (52 - 47) / (100 - 47) = 9.4%. That's a big number, but I don't know that it's big enough to be the death of many cities - it seems plausible that those 9.4% could move back out over the next few years for the same reasons they moved out in the first place. (And remember that young people living independently are only a part of a city's population.)
Of course, it's likely that this impact isn't going to be uniform, so that doesn't mean individual neighborhoods (or even individual cities) have no cause for concern.