And if not enough leave?

Realistically, the senior Captains are getting plenty of vacation, fly good trips and can drop their trips if they want time off.

Why would they leave so they can buy their own insurance and take a 100% paycut?

Sure - I bet Delta will give them as an early out 6 months of pay or so, maybe $20K in medical. But like George in Seinfeld said, "if I stay, I get it all" - 12 months of pay a year and subsidized medical.