I can say that my experience has been different from Smarta$$. Most captains I fly with are staying, unless they are junior and young.
However, I have yet to meet a copilot that hasn't got apps out there. The universal response as to why doesn't even include pay, benefits, etc. It's always the same thing. "In the next 10 years I'm going to move up about 100 numbers." No movement. While at every legacy, you'll move up 100 numbers in a few months.
The math says go if you are in the right seat, it even says go if you are a young enough and junior enough captain. I think most guys realize this, and once the hiring starts bigtime, it's going to get real ugly.