Open recursors asking for random subdomains can generate bigger volume of attack, but still, they are smart and will fall back if the server is overwhelmed.

Even if you're assuming 100 qps from each of the 94k recursors, that's only 9.4M qps. And most of the recursors will notice lack of answer and will slow down / stop the queries. In practice random subdomain attacks rarely generate more than a million qps (YMMV, there are exceptions, technical nitpics, etc).