Here is a thought, which I think I have evidence to suggest is plausible:
Is it possible that Google has adopted a mechanism to fight Googleating and
content spam and does so by fixing and imposing some limits on the number
of referrals it will endow a site with?
I know that this may be impossible as SERP's are not directly correlated to
the number of referrals and hence cannot balance them. Still, I think there
must be some mechanism -- something beyond PageRank on its own -- that
ensures sites are not customised to attract maximal traffic.
As it's a factor that often ruins Google as a search engine, would it not be
rational to assume that one of the ways to combat content spam is to have
this 'patch' added to the algorithm?