I certainly can't think of any either. In every case I can think of, the HC did fall off - but not completely **** the bed the way Belichick did.
Lombardi left GB for Washington and gave them their first winning season in 15 years. The future was bright before Lombardi's tragic death that offseason.
Bill Walsh doesn't have any "after Montana" experience at the pro level.
Tom Landry and Don Shula have very similar career trajectories. They each won a pair of Super Bowls and kept coaching afterwards for years without winning one again, but were very much in contention more than once.
Chuck Noll was still respectable after Bradshaw left. Like the above there were no more championships, but there were a couple playoff wins in there and a trip to a CCG.