Warning: This is a long post. TLDR at the end.
Or, more accurately, the case against trading Jaylen Brown for James Harden. Lets say the Celtics were to complete a trade for Harden involving Brown, the fundamental tension in that deal is you are trading future value for current value. Brown is 24, Harden is 31, the bet you are making is that by acquiring Harden you are getting good enough in the immediate future to win it all thus negating the likely surplus value in Brown will have over Harden in future years given his age, contract, ect. So any discussion of the deal has to start with Harden's contract situation and age.
Harden is under contract for 2 years with a player option. If he plays at his elite level the next two years at ages 31 and 32 then he's opting out an getting a new contract at age 33 where he'll be eligible for a 5 year max starting at 35% of the cap. A massive deal for his year 33-37. That is almost guaranteed to instantly be one of the worst contracts in the NBA upon signing, even if he's still very good the he's unlikely to be 45+ million dollars good even in the first year of that contract. And if he opts into the third year, it's because something went wrong in the first two (injury, decline, ect) which is not ideal. So really with Harden you're looking at a two year window after which age related decline, burdensome contract, or just plain leaving via free agency decrease his expected value significantly. Whereas after those two or three years Brown will be hitting his prime, likely still be paid less, and have several years where he will likely outperform Harden on the back end.
Now if you can win it all the next two years that might be worth it, but can you? A major factor lost in all this is it will be extremely difficult to build the team year one into a contender. You'll be acquiring Harden during the season where there will be NO practice time due to the game load. Chances to build chemistry off the court will be limited. But also the Celtics are hard-capped. A straight trade of Brown for Harden pushed you VERY close to that hard cap and into the tax. Of course it wont be a straight trade, but every asset you give up in addition to Brown hurts your ability to construct your team. If Marcus Smart is in it you lose a second starter, if he's not then one of Theis/Thompson almost has to go to make the money work (because Harden makes 18 million more than Brown), and you're likely stacking young guys + draft picks. Every player you include opens up room under the hard cap, but not that much because most those guys are making no more than 3 million and you have to replace those guys on the roster in the middle of the season which is REALLY hard to do. You may have the TPE, but again because of the hard-cap you wont have a lot of room to use it and you'll be out assets from the Harden trade making it hard to use effectively anyway. All of this is a long winded way of saying if you think the Celtics bench is bad now its gonna absolutely blow after a Harden trade.
So okay, your bench sucks. Then other problem is as good as Harden is you still arent going to have the best player is a Durant/Nets series (if he looks like he has so far this year), or a Giannis/Bucks series, or a Lakers/Lebron series, or a Clippers/Kawhi series. An you likely have to get through 3 of those 4 to win it all. "Okay, but Jaylen doesn't help you with that either!" you may say. Sure, but a Jaylen for Harden trade is shifting value to now, and the bottom line is given your bench and Hardens inherent shortcomings (ie he is bad defensively) you arent likely good enough year one. Maybe you are if Kemba is totally healthy, but thats a BIG bet to make. And even if he is there are inherent fit issues with having three ball dominant guys on offense and a defensive backcourt of Kemba/Harden. You're probably best off trying to flip Kemba for pieces, but again that SO hard mid season.
So really your bet is that with a full offseason Ainge can build a championship bench and NEXT year is your year. Of course the team is now VERY expensive with Harden, Tatum (hopefully at the 30 percent max) and Kemba, making it hard to fill out roster (You wont have the full MLE for example). You don't know if Kemba will be healthy. Tatum will be 24 by NEXT years playoffs, so he should be hitting his stride by then but you'll also have a complete CIRCUS all year with Harden facing free agency. The last time the Celtics faced that it... didn't end well. One injury to Tatum/Harden and its complete disaster. Game over.
So what if you don't win the next two years and Harden does leave? We've certainly seen guys leave here a lot the last few years. And its not like Harden would be sticking around for Boston's superior gentlemen's clubs. Well now you have pretty much just Tatum, you've stripped at minimum a fair amount of assets off the team, and thing could get very dark. Like Tatum asking for a trade dark because the roster is now barren.
If all that is too much for you think about it this way, the best chance of winning a title is to have as many elite players peaking at their primes at the same time. The reason Tatum/Harden dont make all that much sense is an eight year difference in age all but assures that wont happen. By the time Tatum is in his prime Harden is well into decline. Tatum may not be good enough the next two years to push Harden over the top while Harden is still in his.
Now for a Jaylen Brown note. Its been two games, but WHAT IF Brown is closer to the guy he's been in those two games then the guy he was last year? A 6'6 freak athlete who can get his own shot at all three levels, defend 1-5, AND play-make for others. He's been their best player. He's running pick and rolls smoothly, making skip passes to the corner, and juking guys out of their shoes. If he plays at that level he's not only an all star, he'll make some All NBA Teams in his career. If Brown's upside in now High Level On Ball Creator and not just Elite Secondary Star then honestly, I think it might be crazy to trade him for a 31 year old. We dont know he's that, it's only been two games, but man thats a tantalizing possibility. It closes the gap between Brown and Harden's value in the near term, while further widening it in the longer term making the deal harder to justify.
Now all of this is just basketball reasoning, i think you also have to seriously consider the "intangibles." Harden by all accounts seems like a guy players dont really like playing with all that much. He hasn't exactly showed himself in a good light the last few weeks. Stevens didn't exactly demonstrate an ability to manage guys like that with Kyrie. What if Tatum just hates him, or doesn't like the reduced usage that comes with playing with him? Do we think Harden is truly committed to winning? If we did resign him do we think he will age well given his lifestyle of partying? Remember, we are trading an ELITE character guy for Harden, a guy who gets better every year. And just as importantly a guy we actually like as a human being. Harden can be...hard to root for.
TLDR version: I dont think Harden makes you good enough in the short term to sacrifice the longer term value of Jaylen Brown given factors such as his age, team construction, competition ect. I think there's some chance Jaylen is about to explode this year and become a true On Ball Playmaker which would close the short term value gap. And I think the intangibles make Harden a less than ideal player to make a big bet on. I'd pass.