I would rather compare PHB2 to 3.5 PHB instead of "complete warrior".
I feel like these chart games we are having here are a bit stupid (my dad is stronger than yours or something like that)
There is some logic to it. The question is whether 4e is a "failure." The only definition we have of success is 3.0/3.5 (no one would call that edition a commercial failure). So it seems like our best way to guess at the "sucess" of 4e is to compare it to 3e.
Wasn't there a 3e PHB2? Did that make the best sellers list? Did any 3e books besides the core books make the list? If not, that would be strong evidence that the 4e strategy of serial core books is working.
(I acknowledge that there are other possible explanations such as 3e having more robust competition if non-fiction books in general were selling more in the past or for similar reasons).