All that really matters is how many Captains there are, and how many FOs that can upgrade quickly.

The real number is how many line qualified Captains does each regional have. How many Captains that bid and fly the line every month.

In the next 18 months, we could "half" the regionals.

The problem is that when you "half" the regionals, you are taking the top half off as the majority of the hires will be captains.

For easy math, let's say there is a fictions airline that has 1000 actual line captains (with medical, not on leave, and actually flying every month.) Then, in a 5 month span, they lose 200 captains (40 per month). The airline will start to park airplanes because they can not staff the flying. Thus, fewer FOs will be needed. They will do everything that they can to hire street captains and to upgrade all of the FOs that are eligible. Huge bonuses for street captains (as you see now at several carriers) will pop up, and the airlines will do everything that they can to prevent pilots from leaving (huge retention bonuses, as you see now).

Then they lose another 200.

Then another 200.

The attrition will be faster than FO's can fly 1000 hours and upgrade to captain.

Think about it. It takes at least 2 years of busy flying to really become a captain (initial training, IOE, building time, then upgrade training, OE). In the matter of 1 year, the regionals could lose +6000 captains, and at 18 months, lose closer to 10,000. Losing captains that aren't replaced means fewer flights, which means fewer FO's building time, which means fewer FO's eligible to upgrade in the future. It is a vicious cycle.

If the regionals lose 10,000 pilots to the majors, they will be much smaller than 1/2 the size that they are now. They might be a 1/4 of the size. The cycle has already started, as is evident by the huge retention and street captain bonuses.