If they were so flexible, they would work from a condo overlooking the ocean in a tropical island. There is little need to congregate in an old city for this kind of work.
Because jobs aren't flexible. What I mean is that the people that could move somewhere more "fun" or "interesting" because they could get a relatively high paying job to cover the rent did, and there's no reason for them to stay if those jobs disappear. I could imagine a lot of "reverse gentrification" in insane rent markets like SF or NYC.