No it isn't?
We know nothing about the Ancient kingdom.
We've been told that D's are very rare in the world. The wg saw to that.
They
are rare
A handful of characters in the entire series having the D. in their name doesn't make it any less rare
It's like saying CoC isn't rare when there are 20 (?) characters in the entire series with it
And it also makes the original/first D's in the story way less special.
Oh Roger's a D? Guess what? So is that homeless dude by the beach.
Everyone in a family gains the D.
Law's sister Lammy was a D. along with whichever parent he inherited it from and any siblings they may have had
Everyone in the Nefertari family is a D.
Any relatives Saul may have carry the D.
Having the D. in your name isn't some special title that you earn
It's a name that's passed through entire family lines
So when every Dick, Tom and Harry turns out to be a fucking D, the already nonexistent competence of the WG goes down further.
The WG doesn't have a special device that allows them to trace your bloodline
Are they supposed to find
every D. in existence and kill them? It's a secret name for a reason
If you don't tell anyone how would they find out? Magic?