On a flight to Orlando, the next flight isn't going to be next week, but it might be a week before you get on a flight. There are plenty of first hand reports of people being stranded by ULCCs for multiple days. It happened to a colleague of mine a month ago flying LAS-CVG, his flight ended up cancelled for a mechanical and was told they could confirm him on a flight in three days. The new CEO of Spirit famously left 150 passengers stranded in Mexico when he was with Sun Country and their last seasonal flight of the year was cancelled. The passengers had to make their own arrangements to get back to MSP.
Spirit and Frontier operate rescue flights when they are forced to land at places they don't have regular service, like the Spirit plane last week that was forced to land in Bermuda due to a massive fuel leak. They don't have the crews or planes to operate rescue flights when they cancel a flight.
Completely agree that most of the complaints about the ULCCs are people that don't understand what they signed up for. That doesn't change that there is a very real question of what happens when things start going sideways. Just like your car rental with Fox, when things work as expected, they're fine, but when things go sideways, there is little chance for recovery. My personal choice is that I wouldn't use them for anything that I would be horribly disappointed to miss, and I won't use them for business travel. One of the organizations I contract with tried it and it bit them. They saved $100 on the flight initially, then had to pay $400 to get my colleague home the next day.
If I'm flying Delta DTW-MCO and my flight gets cancelled, there are four other DTW-MCO direct flights and about 15 other options connecting through ATL, MSP, LGA, I think I can even get there through CVG, DCA and probably SLC and still make it the same day or at least within 24 hours. If my Spirit flight gets cancelled, it could be two days until there is another one and there may not be space. If for some reason I can't get on any Delta options, I can push Delta to sign it over to AA or UA, which can route me through ORD, CLT, DFW, EWR, DCA.