The Warriors are well set up with all their key players young and under contract. Barnes might be a tough re-sign and Iguodala probably will slow down in a few years, but they aren't impossible pieces to replace.
Besides, it's not just about matching up with the Warriors. In the Eastern Conference you need to beat the Cavs and they will be solid as long as LeBron is still playing well, maybe even a little past that when you consider Irving and Love are still young. They present a lot of problems for the Celtics with their size, depth, and scoring.
I don't understand why you think they will become "less savvy" or "less smart." It stands to reason that at their age, Green, Curry, and Thompson will only improve with more experience. Curry and Green have already shown improvements in their game this year as opposed to last.
If you want to say that their success might be fragile considering it largely relies on Curry who, as a smaller player with a history of ankle injuries is more easily banged up, I suppose I could buy that. All teams are largely dependent on their star's health though.
OK, so Curry and LeBron are not Larry Bird. Did I miss the news bulletin that we signed the next Larry Bird? They're the best in the league right now and historical comparisons aren't really relevant.