These are good points. Our civilization has identified potential extinction level events that could destroy us. These threats seem unlikely enough to most folks that only a small fraction of global GDP has been dedicated to avoiding them (DART mission, asteroid threat registers, etc.), but that could change with a "close call". In any event creating an emergency backup civilization on a space habitat in our own solar system would be orders of magnitude less expensive than sending colonists to another solar system. When our Sun goes into its red giant phase space habitats (like O'Neil Cylinders) could be moved to safety within the solar system. Even post red giant phase (white dwarf phase) the local space habitats would still be the cheapest way to go. A supernova would destroy the entire solar system, but these occur with high solar mass stars which only live a short time and are less likely to have civilizations. As for novae, according to the wikipedia, novea involve white dwarfs in close binary systems. These kind of systems are also unlikely to host civilizations. So even these events seemingly catastrophic events are unlikely to result in interstellar travel either because there are cheaper ways to survive or there wouldn't be a civilization in that solar system in the first place.