You have to make that deal. I love me some Tatum, but this is some "do we trade Al Jefferson for Kevin Garnett" level stuff.
Giannis is an MVP level player in his prime. Tatum is a really good prospect who might get there one day. You trade a prospect for a top-5 player in the league 100 times out of 100.
Tatum, Smart, Morris, Yabusele, LAL18 (let's say #4), two more 1st rounders... and MIL probably still says no.
Irving / Brown/ Hayward/ Giannis core would have a 5+ year window.