This is an excellent point that you make and I do agree on principle. My issue is really more with Trae and how his play style translates with regards to maximizing the team and translating to the post-season. From what I've seen with Trae, he seems like he excels in the two man game and he would definitely turn the spurs into lob city with the easy buckets he can get Wemby. But my impression of him is that he's had a lot of struggles playing off-ball and you're basically buying into a heliocentric offense with him on your team. I don't know if you're giving Wemby or the rest of the team an opportunity to grow much with Trae's play style. His shooting percentages fall off a cliff in the playoffs and he can get hunted pretty relentlessly due to his size. None of this matters much in the regular season, I think with the right team around him and Wemby he turns the Spurs into a 55 win regular season juggernaut season after season but for a guy with as many weaknesses as he does it's a high price to pay for the (at least) 3, more likely 5 draft picks. You're basically committing to Trae and Wemby as your core for the next 5 years plus. I think I'd feel a lot better about it if I didn't see such a dramatic drop-off in the playoffs.
Again, I do agree with you that 3 Devin Vassells do not equal 1 Trae Young, and from an expected value standpoint Trae is worth more than 2 unprotected Atlanta picks and their swap, I just think that 1) it'll take more than those 3 picks to get it done and 2) I don't know if Trae is that guy to be going all in with.