Computers
It is not "a computer." On the FBW Airbuses, there are five Flight Control computers. They are designed and built by two independent companies, to avoid the duplication of any software glitches. For one computer to fail is rare; for two computers to fail on the same flight is rarer still. Even if all five were to fail, the aircraft can be flown manually, via the elevator trim for pitch and the rudder for roll. Sluggish, of course, but manageable and practised in the sim.