Originally Posted by
3Cforme
It's not strictly an LCC problem nor Frontier problem. Delta has served a number of Caribbean destinations just one a day or even just once a week. Delta isn't going to put you on American if they have a mechanical.
But Delta could put you on United, and even if they can't, you'd probably spend a few hours or a day there while Delta ferried in a rescue plane from ATL or something. ULCCs often do not have spares.