Ainge got this team back to being one of the upper echelon teams in the league. The NBA is going more in the direction of MLB than the NFL - a league of haves and have-nots. We have an ownership group willing to spend to succeed, because they know success will bring in revenue (something that just isn't guaranteed in a lot of other cities, making bigtime spending impracticable). We have a GM who, while I have criticized him in the past for being fickle and sometimes disingenuous, is creative and aggressive about getting talent on the roster and wants to win. Those are two key factors in continuing success.
Don't forget that apart from bad luck and bad drafting in the 90s and early 00s, the Celtics were crippled by terrible ownership from Paul Gaston. Bad, meddling ownership and incompetent management sets the tone for continuing futility, as seen with franchises like the Clippers, Grizzlies, Timberwolves, Hawks (yeah, they had a decent two year run, but they've had a terrible few decades), and Knicks over the past couple decades. Boston no longer has that.
And when you detail what we had and gave up, it really wasn't much. The Celtics are not a team that relied on multiple high draft picks or huge free agent signings. This was a team built through smart draft decisions outside the lottery (Pierce the only player on the roster drafted by the Celtics in the lottery) and good trades. That can continue.