That was true, but it's not true any more. Modern targeting pods (like the Siper XR pods on the B-1) mean you can get the same situational awareness from much farther away. Between targeting pods and laser guided weapons the A-10 is obsolete.
Low and slow is dangerous. Aircraft defenses (lasers, flares, chaff, ECM) haven't been evolving as quickly as threats at low altitudes. Not by a long shot. You're fine against a dozen guys with rusty AK-47s and RPG-7s, but once you start fighting armies equipped with something like Pantsir or NASAMS you're in big trouble.