Spurs made a wrong decision.
Leonard is a special player, while DeRozan is good but inconsistent, this is one of the big mistake the Spurs made in their franchise.
If they have traded Leonard to someone else, a star of course, Spurs might still has a good chance to be in the playoff spot now.
Spurs didn't have much options, they simply took what they could for Kawhi, as he simply wanted to leave. Pop even went to his home trying to talki him into staying, offering him 221 million USD for 5 years, money that no other team could offer him, and yet he didn't want to stay. Just to show you how much money he passed, he is currently on 3/101 million USD contract.
The other important thing that was issue for Spurs is that he was entering his last year of contract, and those players have lower value, since you never know whether they will stay or not. So let's say you give your big star for Kawhi, it doesn't work out, and he leaves your team after 1 year. Toronto gambled, they won the title vs GSW that was gimped with injuries, and it paid off. And after that he left them.
So, Spurs didn't what they could, but unfotunately, when player want's to leave, you can't do much.