First, both our offensive and defensive schemes are complex, and take time for the players to settle into. The offense improved from years previous, but due to a multitude of factors the defense regressed. Fortunately, defense is what Rex is known for.
Second, of the 12 playoff teams last year, the average tenure of the HC was 6.33 years and the median was 5 years. 3 second year coaches made the playoffs, but 2 of those 3 teams were in divisions that had no other teams with a winning record (HOU & WAS). If NE has taught us anything this millennium it is that consistency at HC and QB, combined with an established scheme, trumps talent.
You also bring up his W/L record prior to being hired as if it has any bearing on his current tenure. Lot of factors go into W/L including Coaching, GM, Roster, QB, etc. Rex's record in his time with the Jets was 46-50 (0.479), and in his first year with the Bills their record was 8-8. Belichick's HC record prior to joining the Pats* was 36-44 (0.450) and in his first year with the Pats* their record was 5-11. Am I saying we're going to go 11-5 and win the Super Bowl behind our 6th round QB in year 2 because Rex is Belichick? No, of course not, but I also doubt you would have been looking to run Belichick out of town in his second year. Let's just see how things play out and see if the schemes continue to settle in and expand.