If a company isn't paying its pilots enough for them to willingly man their flights beyond what they're scheduled for, they either need to 1) make it more worth their pilots to do so in terms of pay and benefits or 2) need to hire more pilots... both of which cost money.

A company like Spirit posting large profits can't cry poor mouth and seek concessions from its pilot group during negotiations in a pilot deficit when other companies are freely giving their employees pay raises and increases in profit sharing and others are paying upto 300% premium pay to entice their folks to pickup flights on their off time.

It's very simple, if you pay they will come... if you don't, they won't, and you'll have to close up shop...