Their free DNS is only a small piece of the pie.
The fact that 30%+ of the web relies on their caching services, routablility services and DDoS protection services is the main pull.
Their DNS is only really for data collection and to front as "good will"
> The fact that 30%+ of the web relies on their caching services
30% of the web might use their caching services. 'Relies on' implies that it wouldn't work without them, which I doubt is the case.
It might be the case for the biggest 1% of that 30%. But not the whole lot.