If Brown keeps playing the way he is playing, then I don't think it will be much of an issue either way. I agree that Tatum is probably the better talent, but a Jaylen Brown who reaches his ceiling is no slouch.
Still, a package of (a new and improved) Brown, Smart, blah blah blah, Sac pick, and Mem pick is still ridiculously good. Perhaps Boston is willing to give up an additional 'prime' pick (both Mem and Sac) in addition to Brown, so that he can keep Tatum.