You don't need to keep your borders closed forever. You can allow outbound flights without restrictions and make everyone on inbound flights go to a centralized quarantine facility for 2 or 3 weeks + test them.
What about cargo? Better yet illegal cargo we can't seem to get a grasp on. Or illegal border crossings? It only takes a single thing/person to kick off a chain reaction that gets us back to here.
Cargo isn't believed to be a significant transmission vector and has almost universally been excluded from border closures. If a country wants to be very cautious they can mandate that cargo sits in a warehouse for a few days, although that was almost always going to happen anyway.
