I'm not defending Mayhew here, but I think people are doing a bit of wishful thinking when they analyze Mayhew vs other GMs. Much like free agency, where we tend to think guys coming from other teams are great because we've only seen their highlights (while we see all the good and bad for Detroit, so we know the relative merits of each player), I think people are viewing other GMs too positively. There was an article on FiveThirtyEight recently that showed that pretty much every team is hit or miss with their draft picks. The only GMs in recent history to buck this trend over a 3 year period were Ron Wolf (Green Bay), A.J. Smith (San Diego), and Bill Polian (Indy), and even they were fired after their teams went downhill.
Link is here: http://fivethirtyeight.com/features/no-team-can-beat-the-draft/
My point is that when we think about possibly replacing a part of the Lions organization, we tend to have a "grass is greener" mentality. I think it stems from the fact that we are paying close attention to the team. But if we paid as close attention to the other teams around the league, we'd see that almost nobody is doing it better. It doesn't mean that Mayhew should keep his job if the team doesn't produce. It just means that his performance probably isn't quite as bad as we think, and getting someone else most likely would generate a similar level of performance.