Surely the point of this thread is to analyse the likely future size and shape of regional aviation, hence which airports will survive. Not null at all.
s7s's classification provides an exellent starting point. But commercial logic, not the logic of passenger numbers, will determine the outcome. Thus a megagarch who is willing to pay to keep an airport open for her one flight a year will get her way. I exaggerate but you catch my drift.
So, the regional / 'marginal' airports whose catchment areas include enough business aviation / people willing to pay for premium flights close to their McMansions, will survive, subject to competition. Likewise, the island airports etc. for which there is no economic alternative will stumble on, perhaps with subsidy. The owners will find alternative uses for the other airports.
I reckon most or all of the major regionals will survive 20 years hence in some shape or other, though some might be much smaller than today.