You give way too much considerations to the qualifications of a GM candidate . Nothing can guaranty you what kind of job he will do . What makes a candidate better than another one, isn't something that clear . And the marge between candidates is really thin .
The actual GMs are the ones that were fired yesterday , or a guess a team takes on a newbie . The ones that were genius few seasons ago are now the ones with a team at the bottom of the league . There are a lot of things that could interfered that you can't control all the time ; draft lottery, injuries, divorce, bad relationship , drugs , alcoholism ...
More decision are more guesses than anything else ; draft , player desire to work , health