Relicensing Linux as GPLv3, even if actually possible (GPL is incompatible with everything that's not a subset of GPL, including other versions of GPL: GPLv2 is not compatible with GPLv3) would be unacceptable to many commercial players. It wouldn't be end of the world - most big customers, from RedHat to Android folks already have to maintain their own forks anyway - but it would hurt in the long run.