Do you think that Kawhi is really that good?
His last 3 full seasons, Kawhi has finished 10th, 3rd, and 2nd in MVP voting. So, yes, he is that good. The Spurs went from the 2nd best team in basketball to barely making the playoffs and the only real roster change was Kawhi not playing. So, yes, he is that good.
Okay, what's your trade then Moranis?
What are you willing to give up for Kawhi?
Any number of packages would work fine for me. In some ways it makes more sense to keep Brown than Tatum, because Brown can play SG and I don't think Kawhi, Hayward, or Tatum can, so Tatum would get squeezed and the team would be starting someone like Smart. Whereas if you traded Tatum, but kept Brown, you then have Brown at SG, Hayward and Leonard at the 3 and 4. That said, I'd only give up 1 of Tatum or Brown, and would only include Tatum if I had assurances that Leonard would re-sign (or at least would be likely to re-sign).
I just don't think a team like Boston should pass on a top 5 player that has only just entered his prime on the off chance he might leave in a year. I'd go all in and use the season to convince him to stay (and winning sure does go a long way towards that).