Actually a "market" should lead to people playing in the leagues which are "end of the ladder" for them over time. And now that BB is around for a couple of years, most players fit into that category.
In German BBL, there´s like 75% of the players not really improving anymore too. Actually, outside college leagues, I think that´s pretty normal. Only the young guns -really- improve, while the old ones settle for what they do best or improve a tiny bit here and there. If they have room for improvement, they usually change league to a stronger division or league sooner or later. So the majority of the players around are "end of the ladder" or kinda "playing for result, not for improvement".
Zwei Dinge sind unendlich, die Dummheit und das All...