career progression, just do the math, @DL -13 years to 50% of the entire list, UA 12 yrs, US 8 yrs.
these positions will be filled my regional pilots and some military, others, but the part you are accurate about is no pilots are training to the numbers that will be needed on the bottom end, regardless of the low pay. pilots will always jump in for little money, but not enough are even getting their commercial ratings, not even enough for their ATP and 1500 hours to meet the requirements..
Progression for a 30 year old that has been at a regional for 8 -9 years and get hired by DL, will be sitting well under a good contract and at the top of the seniority before the 60th birthday.
SWA and AT, true, that has slowed way down. pilots could possibly leave places like Jb and other smaller carriers for DL, UA, UPS, etc.