I was not saying that every regional pilot is going to the majors in 4 years, just that there will be about 20,000 hired at the majors/lcc's in the next 4 years, and there are less than 20,000 regional pilots.

Other people made that leap. Not me. I know there are lifers at the regionals, and there are those that the majors will not hire.
But it does show that there will be a ton of movement. If you are ready, have a degree, have 121 PIC time, and a decent record, you will be able to move on.

Don't forget that many of the flows right now from the regionals to mainline are guys in their late 50's and early 60's (At least on the AA/WO side of things). According to their message board, Envoy just had a 64 year old flow. Many of the 30 per month that Envoy is flowing to AA will be gone in 5-7 years due to retirement. So, they will fill an opening, and need another pilot to fill that same opening in a few years.

Also, the retirement numbers assume that all pilots stay until reaching 65. Many AA, UAL, and DL pilots are retiring before 65. That may mean that 2019 and 2020 may be much bigger retirement years than they are currently expected to be.