The Transportation Department's new rule forcing planes to disembark passengers after three hours of waiting on the tarmac solves the nightmarish problem of the eight-hour tarmac delay. But one unintended consequence is that airlines may simply cancel flights. That's their current modus operandi whenever a big storm approaches, because they don't want their planes stuck out of position (which is why sometimes airports list themselves as "operational" even when all their flights have been canceled).
There's less downside to simply scrapping the flight: airlines already have the passengers' money, which is why they may suddenly be willing to rebook with no penalty they'd rather book someone onto a new flight than risk having to refund money later on. If you're flying out of a European Union country, be aware that the E.U.'s passenger-friendly laws cover all passengers, even if they're on a non-E.U. airline. So if your flight is delayed or canceled, the airline may have to cover your hotel and meals or now this is a switch pay you a fee.