The OP poses a great question, and this is something I began to address when the KG/Pierce trade was still in the rumor stage. I said it then and I'll say it again: as I look across the NBA landscape, I see no players outside of LeBron and maybe Durant who fit the mold of KG and Ray—not only highly talented (Hall of Fame level), but willing to sacrifice their individual game for the sake of the team, willing to blend well with other big stars for the sake of titles.
Kevin Love, LaMarcus Aldridge, DeMarcus Cousins, Steph Curry, Paul George, Marc Gasol—all very good players, but I don't see any of them being the heart and soul of a title-winning team a lá KG or Pierce. Don't get me wrong; I'd love to have any of them on the Celtics, but I don't think any of them is a "final piece of the puzzle" type of player. And the chances, I think, of landing LeBron or Durant are, respectively, none and nearly none. So I think we have to hope that next year's draft brings us that player, and/or that one of our current guys (Sully, Olynyk, Green) turns into that type of player.
Another thing to consider: KG and Ray joined Pierce, and the only current Boston player who even begins to approach the level of Pierce is Rondo, but will he still be here a season or two from now? And if so, is he good enough to be a Pierce- or even Ray-level player?