I think the Borg know that perfection cannot be obtained. They constantly try to add cultural and technical distinctiveness to their own.
They only assimilate cultures that are worthy of assimilation so it is not their primary goal to assimilate everything.
I do not agree on the humanization part. Fortunately the Borg evolved with the rest of the series. We wouldn't have accepted Janeway with Uhuru's communicator would we?
The Borg's cheap first appearance has been replaced by a look more in line with their viewer ratings just as the mud-monsters of TOS have been replaced by more realistic CGI generated monsters.