By the same logic, didn't and wouldn't legacies hire from the competition's feed to hurt their counterparts?

Ah yes, the dreaded "post hoc ergo propter hoc" fallacy.

Absolutely, but today most regionals have multiple partners so the options are limited there.

Using similar logic, couldn't the legacies just hire all the ULCC pilots to cripple the low cost competition as opposed to actually competing with them? if Spirit had 30 leaving a month it would put a stop to their growth immediately.

Who said they won't do that in the future?

As hard as it was to leave a Captain job to take a pay cut to go to a ULCC I can say that my QOL has vastly improved since leaving the regionals. The lines where I am now average 15-16 days off per month and in Winter months top 17 days off average. Every person will have a different set of circumstances and reasons to either stay or make the move. But one thing is clear. Almost everyone regardless of where you are will have their shot eventually at one of the legacies if that's where they want to be. Whether you're a regional FO, Captain, or ULCC pilot.

With that logic Delta would chase after United's feed and United would chase Delta regional pilots. I doubt either is actually happening. I have had two CPs tell me that yes people are getting hired without the PIC time but they are the exception not the rule.

I also believe JB and SW are far closer to being a major than they are to being Spirit or Frontier. I imagine when SW finally gets their new contract it will be on par with what the major 737 guys are making.

I agree. The QOL at a ULCC such as Frontier or Spirit is better than most (if not all) regionals, and the earning potential is also much more in the long run. That is why I don't think it is a bad move to go to a place like that AFTER you get 1000 TPIC. If for some reason I never get hired at a legacy id rather be stuck at Spirit than a regional. However, if you go to a ULCC prior to getting 1000TPIC, you may have just limited your upward mobility. That may be okay for people who already have limited upward mobility (no college degree, DUI etc.). In short, I think leaving for a ULCC from the right seat of a regional is foolish.

What about hurting your direct competitions regional feed? In this era Delta started doing it in 2012.

There are 3 year captains at every Spirit base. With opportunities for trips being paid on top of guarantee, move up pay, and junior assignments, you can make in the $90s year 2 FO. Thats at $72 / hr current contract. When the new one is signed, I'm betting at least $95 / hr year 2.
I made $117k my last year as RJ Capt. Year 1 is a huge pay cut, but I have serious doubts my regional will even exist in 4 years.