RE: How common is lack of DNS server diversity?

Wouldn't a better term be "authoritative server"? It states what it is,
and doesn't have the semantic overload of your use of "root server."

Unless, of course, you are in marketing in which case you want semantic
overload such as Microsoft's use of "Digital Nervous System" (DNS) to
create confusion.

Root Server == An authoritative server for the "." (root) of the domain
               name system

TLD Server == An authoritative server for a Top Level Domain, such as
              the generic TLDs (COM, EDU, INT) and country code TLDs (CA,
              AU, ZA)