Here's my take. I've done ATP for my MEI and ATP ticket. I've done "local FBO" for everything else except CFI, did that in a University setting. I think that pretty much covers the avenues. I CFI'd for 3+ years at a local FBO, then did accelerated instrument ratings exclusively, and dropped skydivers on the side. I currently drive around the west coast on the left side of a Brazilia.
Regionals like ATP because those guys can pass the training. That's as far as the regional hiring dept. is looking. The hiring dept. is mostly concerned with meeting its hiring goals, and little else.
Real world and real world decisions are completely *completely* different from the structured training environment, and everybody who has been there, knows it.
The beauty of ATP/university training, and Part 121 flying is structure. Lots of eyes looking at every operation. That's great, and is why the safety record is so good.
But what happens when that structure breaks down, or you have dispatchers/maint trying to 'bend' it, or you encounter something you didn't train for? If your whole pilot career has been: here is the paperwork, there is the plane, go fly this profile, you (probably) never had to make a tough decision, or 'push back' against the structure.
If you've done CFI and other single pilot jobs, that don't have the 'structure' around you, you learn to make good decisions, often by having made bad decisions.
Flying on duty hour 13, flight hour 7, with sasquatch in the room above you last night, you got maybe 3-4 hours of sleep, and FO is on their first trip off IOE...and not doing well. Coastal fog is at minimums. Are you safe? You're legal. Dispatch and crew support will say "you're legal". But when will you push back?
The Skydive center owner (with a *serious* anger management problem) wants you to take the last otter load up. Coastal fog is approaching the drop zone. You are *very* hungry for twin turbine PIC time. All the skydivers want to drop. Before you start up, the owner says, "but don't go if you don't think you can drop, I won't pay you for this load if you don't drop them, and I don't want to waste the gas." Do you go? Do you tell them to pound sand?
You look at the fog, and you know it pretty well. It seems to be holding off. You go. At altitude, with the fading sun in your eyes, you have a tough time seeing the drop zone. Is is fogged in, or is that just haze-to-later-become-fog? Experienced drop master says "it looks good. let's go." You know this dropmaster to *always* go, no matter what. All 19 divers are looking at you. Do you give the green light? Or bag it?
Every pilot who has done some *real* PIC flying has a list of stories like this a mile long. That's the key. How much of your flying career have you had to make tough decisions? Going from ATP, buying some ME time with an instructor (also logging PIC), to right seat of that shiny jet, how much decision making happened? When it's time for the left seat, were you paying attention to what was happening on the other side of the plane when you sat right seat? or just said "clear right!, and you're right!"
I can tell you it is a totally different view when *YOU* make the calls.
FA says pax smells of booze. You watched said pax board, and they seemed fine to you, and it's last flight of the night. do you boot 'em?
FO says hydraulic fluid leaking on right side, at an outstation (no mx). You noticed it earlier, and it wasn't too bad. It's day 4 and 2 legs til home. Do you go?
etc, etc.
Which lesson plan from the ATP accelerated training addresses these kinds of things?
They have to be learned in the real world. If you start learning them with 30-70 people behind you, it gets real interesting.
whew, I'm done.