Yes, I'm aware of the monoculture argument. But anyway, the raison d'etre of the GPL is to preserve freedom, not to avoid monocultures, so, even if in this case we might be able to see a good outcome (even though that is still debatable as we have more than one compiler, gcc is big, but far from the only compiler) it's not actually relevant to the question posed in the general case, unless we think the only benefit of the GPL is to avoid monocultures...
Healthy competition. Fresh ideas. Something to turn to should there be an unfixable problem with the other thing.
Monocultures are dangerous and come with a high risk of stagnation.