Could be a bit of selection bias here -- how many coaches have lasted more than six years with a team without winning a title? And for how much beyond six years did they coach?
This might not be a complete list, as I was just doing it manually based on a list of all coaches (and not sorted by years or anything).
Jerry Sloan (23 years in Utah)
John MacLeod (14 years with Phoenix)
Doug Moe (10 seasons in Denver)
Flip Saunders (10 year in Minnesota)
Bill Fitch (9 years in Cleveland before going to Boston and winning in year 2)
Rick Adelman (8 years with Sacramento and that was after 6 years in Portland and 2 years in Golden State)
George Karl (7 in Seattle, 8 in Denver)
Lenny Wilkens (7 years with both Cleveland and Atlanta)
Nate McMillan (7 years in Portland)
Jeff Van Gundy (7 years with New York)
Plenty of coaches made it to 6 years and then for whatever reason were not retained past that year.