I'm not disagreeing with your view on the writers you mentioned, but how are you defining heroic fantasy there? There are stories I would place there that are thousands of years old.
Heroic Fantasy based in a world other than our own. William Morris wrote the first heroic fantasy based in another world. Older heroic fantasies were based somewhere in our world. when i speak of heroic fantasy, i am excluding things like Alice in Wonderland, which is fantasy, but not heroic fantasy...
Of course, i put a lot of stock in the forwards Lin Carter wrote for the Ballantine Adult Fantasy series (released in the late sixties-early seventies). there may be newer, more informed fantasy writer/scholars out there...