Many more people - if things continue as they are - will have to join the ranks of the mobile if they want to prosper or even survive.
As far as I can tell, Florida's solution is an attempt to stem the tide of brain drain. Give talent a reason to stay. Better yet, transform your region into a magnet of creativity. I suggest helping The Stuck become The Mobile. I also recommend serving Mobile interests, instead of catering to The Stuck.
Thus, I have a paradox: If a region encourages mobility, how might it benefit from its investment? What I observe is that places such as "Detroit, Buffalo, or Pittsburgh" suffer from poor demographic churn. Relative to other regions, few people are actually leaving. The deep attachment to place discourages out-migration. The resulting landscape is increasingly parochial, building a barrier to in-migration.
When this happens, the best thing you could do for your citizens is to encourage them to move to improve. Concurrently, the region in question should foster links for these nomads back to home. I figure that these Parochial Argonauts retain an abnormal connection to the place where they grew up.
The city left behind should provide a wealth of frontier opportunities, as well as an excellent reputation for helping people join the ranks of The Mobile. Wherever the jobs are, a Pittsburgher is there willing to lend a hand. That's already the case for many Rust Belt cities. The patterns of out-migration demonstrate this. I can't think of a better way to break into a Cool City and enjoy success.