2X is the absolute floor in regards to relative value
1. 50/50 proposition it conveys (so 2X) +
2. if it does convey, the Celtics receive SA's pick
That’s not the right framework. I’ll simplify. If I tell you there are two sets of three boxes, S-1, S-2, S-3 and C-1, C-2, C-3. In scenario 1 you get an S box and a C box. In scenario 2 you get the higher value of the two boxes but not both, and it’s impossible for the two boxes to be the same:
If box 1 is worth $10,000, box 2 is worth $1,000, box 3 is worth $100, the expected value of the swap is much greater than 50%.
That’s because you are stacking odds of the best outcome, which is much more valuable than the other outcomes:
S-1, C-2: swap gets you $10,000 and trade gets you $11,000.
S-1, C-3 swap gets you $10,000 and trade gets you $10,100.
S-2, C-1: swap gets you $10,000 and trade gets you $11,000.
S-2, C-3: swap gets you $1,000 and trade gets you $1,100.
S-3, C-1: swap gets you $10,000 and trade gets you $10,100.
S-3, C-2: swap gets you $1,000 and trade gets you $1,100.
The expected value of the trade is $7,400 and for the swap is $7,000. It’s not half is valuable because value isn’t linear so the improved chances of the best thing are much more valuable than the lost piece in the swap. Even if you do this as just the value of the C box being give up, the vastly greater value of the highest value box is critical. In the trade scenarios, Boston would trade away an average of $3,700 in value and get nothing. In the swap scenario they’d trade away an average of $3,500 in value and keep/receive an average of $400 in value.
Now, my example is designed to prove the point and obviously value isn’t shifting by orders of magnitude in the draft in the same way it is in my example, but this still shows the flaw in your logic. Put differently, it matters a lot more that if the Celtics get the second pick, the Spurs will almost certainly take it than it does that if we get the 26th pick there’s a reasonable chance we keep it or that they have to give us a pick back if we get the 2nd pick.
To be clear, I strongly support the trade and don’t say this to argue that it is risky, just to clarify that the way you are thinking about the risk is too simplified and therefore wrong.