Honestly, I feel bad for Mayo. By all accounts, he was a great guy and well respected by his teammates (when he was one of the captains) and players (both as defensive assistant and head coach). But he was also clearly in over his head as coach. He likely would have benefited from being the head honcho on defense (aka, true DC) for a couple of seasons, but circumstances seemed to get ahead of everyone. As many have noted - unforced error on Robert Kraft's part.
But, such is life in the NFL. Despite what some of his detractors on this forum will claim, Mayo will get assistant coaching opportunities at the NFL in short order, and possibly NCAA as well. And probably should get another opportunity as head coach at some point (just not next season).