I've never thought so much about assistant coaches in basketball. I doubt it makes much difference at all, with some historical exceptions for revolutionary minds.
I think you're right 90% of the time, but the Celtic's Coaching situation was unique this year:
Popular HC, off the NBA Finals, is mysteriously
fired suspended with players questioning why? +
National NBA media PR blitz questioning St. IME's suspension? +
Joe is only 34yrs old +
He's a rookie NBA HC +
Joe has never been a lead Asst in the NBA +
He starts a day before training camp +
The previous lead asst leaves for HC job (no replacement found) +
Damon, next lead asst, leaves 2/3s into the season (no replacement found) +
All the rest of the Assts walk out the door days after the end of the season +
Those Assts primarily go to a shite team like the Rockets because they are loyal to IME +
The pressure of Championship favorites all season
Joe had no say on his staff and was thrust into an incredibly sticky situation. Finishing with the 2nd best record in the NBA and making it to G7 ECF is disappointing, but shouldn't be shocking, especially losing to a well-coached, super-hot Miami team.
Hogan's Silver Linings Coaching Post