Worst case scenario: we get one year of Kawhi, he's average, we don't get past the 2nd round, he leaves in FA summer 2019, and we still have Dame, Nurk, Collins, Moe, and a decent bit of cap space/flexibility a year from now. That's probably no worse than where we are right now.
As for if Boston's assets are better: if Ainge had a stroke and offered Tatum and Brown for CJ and Turner, I'd do that in a heartbeat. I'm sure the Spurs would much rather that pair than ours. Salary matching is really the only question, and would be Boston's biggest hindrance in making such a deal.