Of course it is fair. It is also fair to ask the same question about Pop now that he doesn't have Duncan. You have to win in the NFL and Bill is a loser without Brady. He has some leeway of course because he won so much with Brady, but at some point soon he needs to start winning without him.
Yes it is fair. Bill is definitely hurting his legacy right now.
Was it fair to ask Asante Samuel, the most outspoken critic of Bill Belichick over the last 10 years? Sure. If the rest of the media wants to kiss Bill's footballs, I think it's fair to hear the other side. Especially a former player. I'm convinced Malcolm Butler signed some type of non-disclosure agreement to not reveal what happened before the Philly Super Bowl. Bill has definitely made some enemies, including Ted Johnson and Gronk. He's a jerk (much like Parcells), so it doesn't surprise me that people want to take shots at him.
I personally wouldn't want to have a relationship with the guy (from what I've seen- I've only met him once in person).
Would I want him to run my team? Absolutely. Do I think the league has passed him by? Absolutely. He's 71 years old. Marv Levy retired at 72, and I thought he was a fossil.
I'm ready for the next coach/GM to take this team into the future. All that being said, I've never seen a better football coach. I see no reasons to take shots at him for the past.
I think it's fair to criticize the last 3 years. He's obviously not worth $20m per year, much like Jeter wasn't worth his final contract. This happens a decent amount in sports. At least his number doesn't count against the cap.