A particular topic starts with generalists but as time progresses and stability (i.e. a path worth pursuing) becomes apparent, specialization is needed to eek out more performance gains.
Then after a while, that particular area gets disrupted because there's a fundamental shift in landscape and the cycle repeats because the landscape is new to everyone. Many times this new landscape was brought on because a specialist applied a specific finding to a new, larger group. Then generalists help build that new space.
Maybe this is obvious...