Re: How common is lack of DNS server diversity?

Ah.. your term "root server" is what I have always called "authoritative" e.g. any listed master/slave for a zone.

<Root server> ::= Any DNS server that has final authority for a <domain tier/level>; <domain tier/level> ::= root, TLD, SLD, 3LD, ... nLD (0LD, 1LD, 2LD, ... ,nLD). This is not to be confused with root level servers that have specific authority for dot, at the root level (0LD).