As a Clevelander, I understand why you feel that way but I don't think the concept was "ridiculous" at all. I completely understand why they made the film the way they did. A tri-sport city that's gone 50+ years without a championship is a compelling story nationally, and they make 30 for 30s based on what the American public has interest in.
What would be the point of including "Cleveland success stories"? It directly conflicts with the narrative they're trying to tell. That has zero interest to the viewing audience outside of Cleveland. In my opinion the movie's thesis was "All these terrible things happened to these teams, but these fans remain loyal and die-hard." The end result could have been better, but overall the doc largely succeeded in executing that purpose.
People have to accept that this documentary wasn't made for us...we already knew everything about these stories inside and out, and most of us lived it. It was made for the general public that didn't know the details about these events. This doc isn't and wasn't ever intended to be "A complete history of Cleveland sports since 1964".