I have asked that question a few times in conferences. Capacity wise they can handle it as they already need it to defend against the massive DDoS threat that t

I have asked that question a few times in conferences. Capacity wise they can handle it as they already need it to defend against the massive DDoS threat that they face. Relative recently they also added anycast, which further increases the capacity. DNS queries is also generally quite small and has not significant changed, and the modern resolver software is pretty efficient. DNS is also designed for redundancy so in combination with anycast is quite simply to scale up. Because of the persistent DDoS threat the question of capacity is constantly discussed. I don't remember the exact numbers but the current overhead is quite large compared to actual usage. It is also constantly being increased based on the threat rather than usage.