Okay, granted. I should have said that you have to compromise Google's servers, not specifically its nameservers. The point is the same, that you have to target specific servers and don't get to pick the weakest out of a very large group, so your attack surface drops drastically. Of course, the signing servers aren't going to be Internet-accessible, so will probably be even harder to exploit than the nameservers. But exploiting the nameservers of a huge and well-run shop like Google would already be a pretty difficult feat for even a well-funded criminal hacker group (although maybe not for some governments).