Quite right. The growth rate is unsustainable. In reaction to the loopy 30-year projections which we are inundated regarding Social Security, my ex-colleague Dean did a straight line projection of the prison population and found that we'd all be in jail in 30 years.
That is not to say the present population couldn't be reduced. It means that states will be forced to be more selective about whom they keep locked up. It will make the crime issue less useful to conservatives, since Republican officials will be forced to rationalize a resorting of those incarcerated and a pullback from mandatory minimums for drug offenses.