I guess with hardbop, as with a lot of free music too, I do feel diminishing returns are (and have been) a more regular outcome than they "should" be. For me, it's certainly not a matter of "concept" (that often works only insofar as the concept is played out), and I do feel that there are a lot of gems within hardbop, free jazz, and whatever else. Maybe I'm confusing consistency with sameness, or maybe I just have a tin ear.