Consider salaries when thinking about trading for Leonard.
Right now the Celtics are in a pretty good spot, or at least a manageable one, when it comes to the salary cap. To retain Irving, you'll have to sign him to a mega huge contract starting in 2020/21 (around $40m/year if I'm not mistaken). But the good news is that Horford's last year of his current contract is the year before, meaning that you can let him go in free agency or sign him at a reduced number and still not kill yourself in the tax. Then the next year (prior to 2021/22) both Hayward (unrestricted) and J. Brown (restricted) will be free agents, meaning you have discretion on whether or not to sign one or both. Hayward will be in his early 30s by then, so maybe you re-sign him at slightly less than the max at that point. Then the next year, prior to the 2022/23 season, Tatum is a RFA.
The point is, the Celtics are in a really good spot where they have both prime-age stars (Irving, Horford, Hayward) assisted by rookie contract stars (Brown, Tatum). As the first category's max contracts roll off, you have that second generation coming due. Leonard alters that completely. It's not to say it's a mistake, but you have to be clear that trading for him hitches the wagon completely to Irving and Leonard, especially if you're giving up Tatum in a trade.
The other thing to consider is how a lineup of Irving, Leonard, Tatum, Hayward, and Horford would match up defensively with most teams. I think all three of Leonard, Tatum, and Hayward are closer to 4s already than they are to 2s at this point in their careers, and they'll definitely trend towards being bigger and slower as they age. In other words, you basically have three biggish small forwards coupled with a below-average defensive point guard, which might get eaten up by the Lillards/McCollums and Currys/Thompsons of the world.