They traded a disgruntled MVP caliber talent who hadn't played meaningful minutes for them in over a year for a guy who was selected to the All-NBA second team this year.
What's more, the guy they acquired in the trade can slot into almost the same role as guy they traded away. Granted, he's not nearly as good as the guy they traded, but if keeping Kawhi were an option they wouldn't have traded him.
If you have to trade your star you'd like to get a return that gives the trade a ceiling of what the Celts got out of the Brooklyn trade. In other words, multiple chances at getting a very high pick in the draft.
Failing that, if your goal is continue to be competitive in the short term, exchanging your disgruntled star for a lesser but still very productive and valuable star is pretty great.
The ceiling for this trade is that it allows the Spurs to continue to compete in the 45-55 win range and maybe try to win a playoff series. When you consider that they'd be in the same or worse position in the short term if they'd kept Kawhi, and worse position after next season when Kawhi inevitably left in free agency, that's pretty good.
Given all that, I'd give it an A-. Especially since they also added a solid, productive young big man and an okay-ish pick.