The whole argument of free agents not wanting to come to Boston is unfounded. There is no clear evidence of this.
And adding a few wins to a sub .500 team isn't going to change it, either.
Last year in the east there were only 4 wins separating the 10th seed Miami heat from the 6th seed Milwaukee Bucks.
There were 4 wins separating the 5th seed Washington Wizards from the 3rd seed Chicago Bulls.
The 8th seed (who did, and didn't, make the playoffs) was decided by a tie breaker between Brooklyn and Indiana.
That's just in the East.
In the West there was 1 win separating 6th seed San Antonio Spurs from the 2nd seed Houston Rockets...and the 8th and final seed of the playoffs was determined by a tie breaker between New Orleans and OKC.
The playoffs can get very competitive, and the Eastern Conference this year quite clearly showed how adding a few wins to a sub .500 team can make quite a substantial difference. An extra 6 wins to Boston would have put them in a tiebreaker for the 5th spot. The Wizards (5th seed) actually won their first round matchup and went on to the second round.
If Boston is good enough to get a top 5 seed, win it's first round matchup, and put up a solid fight in the second round (as Washington did) then IMHO that quite dramatically changes how potential free agents view the team.
You can disagree if you like, but that's merely your opinion (as this is mine).