Everyone is assuming the team will take at least a small step backwards if only because you can't go all out year after year and the team no longer has Posey for that extra experience off the bench. But there's one school of thought that says that at least the big three might be even better this year, which might more than make up for any negative influences. See these quotes from Bulpett's article in the Herald.
“I think they all will be better,” Rivers said. “They’re so comfortable with each other. They trust each other. The players trust each other now, and they trust the system. Last year they were learning that trust and this year they start the season with that trust. They’re back again and that’s an improvement. I think that’ll be one of our biggest improvements, not just the Big Three. Our starting five is back, so we should be better than we were last year because we should know each other better than we did last year.”
As good as Pierce, Garnett and Allen were last season, there was a certain feeling-out process as each, the former singular star on his team, moved into proper alignment on a team that had one goal: to win the NBA title.
Now they know better what they’ll be facing. They also know each other more completely.