I think the other point that some are overlooking is that a major failing of this team not only was injuries and a weak bench, but Rivers's mismanagement of the regular season. I think the argument can certainly be made that had Doc chosen to keep Ray and Paul's minutes at 34-35 minutes per game, the C's might've beaten Orlando in the last series. And what's the worst that would have happened? They would have had the third seed. No one else in the East was even close to Cleveland, Orlando, and Boston.
We need to recognize two universal truths about this team:
1) When we decided to have 3 max salaries on the team, we sacrificed our ability to have great depth.
2) When we decided to trade away our youth, we inherited the problem of playing with older players.
That's fine. It's already worked out for us in the form of #17.
So even if Ainge isn't able to upgrade the bench as much as we'd all like, I think the C's need to make a commitment next year to keeping the Big Three's minutes down, even if it means that we don't get the 1 or even 2 seed in the East. We need to go into the year with the mentality that the playoffs are the true start of the season and simply getting an upper echelon seed is enough.
If we do that, even if the bench isn't great, the starters should be ready to play big minutes in the playoffs and a weaker bench won't matter all that much.
Note: I'm not suggesting that we don't need to improve the bench, I'm simply noting that we don't necessarily need Grant Hill, Antonio McDyess, and Rasheed Wallace next year to win #18.