Does it really matter if the cannon is working or not? The A-10, despite its age, is more survivable, cheaper to build and run, can operate out of anywhere (those engines high up ingest less foreign material) and has a much longer loiter time.

One could argue that today's combat landscape is not ideal for the A-10 and that it wouldn't survive against modern air defenses. Which is a very good point. But, if you are going against modern defenses, what do you need a cannon for? Specially an external one which will ruin your radar signature.