at least historically. The only coach in NBA to win his first title with a team after 6 years of coaching with that team was Red Auerbach who won his first title in his 7th season with the Celtics. Al Cervi, Al Attles, Billy Cunningham, and Chuck Daly all won their 1st title with a team in year 6 of coaching with that team. Only 3 coaches won their 1st title after their 11th year of coaching a NBA team overall. Red Auerbach, Bill Fitch, and Larry Brown (it was Brown's 22nd year as a NBA head coach, but his 1st year in Detroit - Brown also had ABA and NCAA experience). Dick Motta did it in his 10th year overall, but as indicated of the 10 year + guys only Auerbach made it past 5 years with that title winning team before breaking through.
Stevens is entering his 5th season, so historically it needs to happen in the next couple of seasons or it isn't going to happen at all.
If I think about this logically it actually makes some sense, because coaches don't win championships superstars do. If a coach is with a team for over 6 years that means they are successful, they wouldn't have gotten a second or third contract without success. If you have had success you are less likely to get a superstar player because you don't have high draft picks. Typically a coach with a superstar is either going to win within 6 years or probably get replaced before they get 6 non championship years and a new coach gets a chance.
If you have a team overachieving for more than 6 years you will keep your job but it's really hard to win without top end talent in this league, if the sport were more variable like hockey or baseball then a team of over achievers can win because of a coach, not in the NBA.
So basically I think there is a reason for these seemingly weird stats but I don't think it necessarily says anything negative about these coaches. Brad has not only been great in the regular season but has really led the team further into the playoffs than many expected the last two years there is no reason to believe Brad isn't a good enough coach to win a title.