It's funny that you started this thread while the winningest regular-season team in NBA history is losing 3-1 in the WCF, primarily because of their opposition's length and athleticism on defense and ability to stop the infamous Curry-Green pick-and-roll.
Well that's really interesting.
So we just had AB get first team defense and Crowder just barely missed 2nd team, and I don't care what the votes say...Smart is the best defender I have ever seen when he isn't being gimmicky weird.
But there is no way on Earth the Thunder trade Durant for all of those guys.
If you are the Thunder and you are forced to trade Westbrook or Durant for one Celtic you are taking Isaiah Thomas. And he is nowhere near the defender these guys are.
Ignoring the chirp, you are both right and wrong, and I respectfully disagree on the premise. All of the great teams can put points on the board like no others; think Thunder, Warriors, Spurs of past years (this year they slowed the pace and moved the ball slightly less), etc. Any team with LeBron on it has the potential to muscle its way to enough points.
The Celtics, as currently constructed, are not one of those top-tier teams. However, once you get to be one of those to-tier teams, championship-level success comes down to consistently stopping other all-world offenses and closing games out. Think the Thunder stymying the Warriors this year or the Warriors shutting down the Cavaliers last year. Heck, even think 1984 Celtics. The whole Finals series against the Lakers changed when McHale set the "no free fast breaks" tone with the clothesline.
The reasons that the Celtics are not top tier as well as common criticisms include, but are not limited to (in no particular order): 1) lack of a go-to guy 2) lack of a 'closer' that can put the team on his back and consistently win isolation match-ups 3) lack of a rim protector 4) lack of shooting.
The Celtics are neither an elite offensive or defensive team. You are right in recognizing that the Celtics have much bigger needs on the offensive end than the defensive, and certainly adding two or three of the "missing pieces" on offense would yield drastic and immediate results. A Harden or Durant type would almost certainly make the Celtics a top-tier team, with their structure and role players already in place. However, you cannot win a championship with one side of the ball, which makes also finding that missing defensive piece, hopefully a big man, all the more instrumental. Offense is certainly more pressing, though.
I equate the Celtics now to the defensive version of the Denver Nuggets from a few years ago. Above-averge on one end, average-to-below-average (for a playoff team) on the other end that overachieves and is never considered a true contender for the title, but with more assets and opportunities to improve.