An often held misconception, but it's really not that simple;

1) There are regionals that fly ALL THREE PAINT JOBS. Although the pilots themselves may not, you'd have a hard time parsing through the app info to ascertain what paint/code said pilot flies. And it's not uncommon to have a pilot fly at least 2 paint jobs on the same "rotation/sequence/trip/pairing/ID"

2) DAL could hire UAX/AE pilots. But UAL could simply hire DCI/AE pilots, and AAG could hire UAX/DCI pilots.

Mutually assured regional destruction. Which would be good. But as a scenario for "crippling the competition", it's not at all the reality.

What SHOULD happen is the hiring agreement for DCI pilots at DAL should be mandated to increase by the DALPA NC. No more metering of the flows that are left or the SSP. Crippling DAL's regional feed, and in the process telling management "ha ha, sure, try and staff those little jets now....."