Giannis is the one (reasonably available) guy I would consider trading Brown for under the knowledge that teams without an MVP level player almost never win title and so trading one non-MVP level guy for an MVP level guy probably substantially increases your chances.
That said I wouldn't feel great about it for the following reasons:
1) I've enjoyed watching our hoe grown guys, to me there's something better about watching players who have been with your team their entire career climb the mountain and win a title.
2) I worry greatly about Giannis and injuries. A 6'11 guy highly reliant on athleticism entering his age 32 season, history is not kind to big man like that. Giannis and Tatum are only a good fit if both guys are healthy and playing, and that's far from a certainty.
3) I worry about the Giannis/Tatum fit. Now on paper it seems terrific, but we thought that about Giannis/Lillard and that blew up in the bucks faces. I do worry both Giannis/Tatum want to be THE GUY with the ball in their hands, and I'm not sure I believe Tatum would be willing to become more an off-ball guy.
4) What else are you giving up? If it was brown for Giannis close to straight up that's one thing, if its Brown plus all or most of your draft picks that's a no for me.
Other ideas like Brown for KD, Brown for Sengun, Brown for different top 10-25 guy all feel like treading water and if that's the alternative I'd rather just watch the guy we drafted and I've rooted for over the last 10 years.