The only reasons not to give up Jaylen for Leonard are concerns about his health and whether or not he'd be willing to re-sign here. Brown is a very good defender and has looked good on offense, but Kawhi Leonard is a two-time DPOY and averaged over 25ppg just 2 years ago. Having high hopes for Brown is great, but expecting him to be as good as Kawhi Leonard is unreasonable
All that said, I don't see us trading for him (either due to injury, not trusting he'll re-sign, or [most likely] the price being too high for us to pay without ripping the team apart or giving up Tatum)