From an economic standpoint, does Spirit honestly care if theyre a revolving door? Theyre the #1 airline in the US in profit margin by a long shot. If they can get an FO to fly for them on first year pay for 6 months and then bail to a major, they would take that deal any day of the week. Their training costs are fixed costs and essentially irrelevant. Don't worry about using Spirit as a stepping stone. Their management secretly LOVES it!