For the last 5 seasons prior to this one, Giannis has averaged 65 games. He is on pace to play 64 this season. This is his 12th season. In those prior 5 seasons, he has been pretty steady at about 30 points and 12 rebs every season. This season he is putting up 31.5 and 12.1.
He is a dominant player currently and projects to continue to be dominant for the foreseeable future, maybe 5 seasons? And after that, in his later 30's, he will still probably be good. He is 30, Brown is 28.
I see no scenario where you don't make this trade if it were on the table (which it never would be). Back to back, three-peat, whatever:
Holiday
White
Tatum
Giannis
Porzingis
That line up starts to rival Johnson, Ainge, Bird, McHale, Parish. It is true that Giannis doesn't shoot the 3 all that well but because Porzingis can, it wouldn't matter. Durability would be the only hesitation. You would have 2 bigs that require minutes management. But in a way, it gives you more redundancy and flexibility to work around it when one or the other is out. It gives the team more options to deal with it, not less.