So Allen needs the ball to be effective, and plays little defense. Pierce dominates the ball, and plays little defense. Garnett doesn't score inside, so Boston's inside scoring got worse (although interior defense got leaps and bounds better). They still do not have a PG, they still do not have any semblance of perimeter defense, they still do not have decent coaching.
Frankly, I would be surprised if Boston comes out of the 2nd round, even in the East.
EDIT: I forgot to mention their bench.
EDIT 2: You can't put 3 superstars on the same team, and expect them to average the same amount of points they did the previous season. I would expect Garnett to average a career low next year.