We're unlikely to ever actually run out. Ethernet addresses are expected to be universally unique, but they're only required to be unique within a collision domain. If someone started reusing addresses from 3c503s, chances are high nobody would notice. If we did run out, devices would need to start generating randomized addresses, and maybe probe for collisions, which isn't unworkable; the number of nodes in a collision domain tends to be low, and the space is large, you might only barely need to to probe for collisions at all if you have a good random source.