Serious question: Why?
You get one year of Kawhi, assuming he tests the market next summer (and maybe he doesn't, but everything that's unfolded this season would lead me to question that). Or you can have 2 years of Brown at wayyyyy below market value, followed by matching whatever RFA contract he signs, which likely means 6-7 total years of control. I get that the team would have considerable advantages to retain Kawhi with Bird rights, but why introduce the uncertainty, especially when it's compounded by the personal uncertainty around Kawhi's specific situation? If you believe in Brown's potential (and I do!) (and Ainge does!), why would you pay to upgrade for one year at that sort of cost?
You're not making that trade without an extension.