While Chamberlain scored big, Russell won all the championships, by having his teammates do the scoring while Russell did the defense and rebounding.
Observing this, coaches learned that the hub of a wheel shouldn't hit the road; it should enable the rim around it to do so. This is still the conception of how a center should play.
So by the end of the 1960s decade, Chamberlain's coaches changed his style to emphasize scoring less. Near the end of his career, former Celtic Bill Sharman coached the Lakers to 33 wins in a row by having Wilt average only 13 ppg.
My point is that his scoring decrease was caused by a change in the experts' philosophy (conventional wisdom), not by a dropoff in his abilities.