Complex sentence: For many topics, I do not believe that a can be achieved through the wisdom of crowds, since I do not encounter crowds of editors, a perception supported by the studies that show a steep decline in the number of active contributors.
Split into simpler sentences: For many topics, I do not believe that a can be achieved through the wisdom of crowds. Also I do not encounter crowds of editors, a perception supported by the studies that show a steep decline in the number of active contributors.