They got a better player in Kyrie.
By how much, especially when one considers things like leadership and team basketball? Plus, Dallas parted with a lot of draft capital.
How much ? A lot.
Brunson could not be as effective as he is in NY without having the ball in his hands much much more than he would be able to do so in Dallas next to Luka.
It remains to be seen just how good Brunson can be when sharing the ball with legit stars instead of a mediocre cast like he has in NY. He was brilliant in Dallas too when Luka was out but his numbers went way down next to Luka. I believe there is some serious stat-inflation with Brunson in NY right now.
In contrast, Kyrie has shown himself to be very effective next to LeBron, Harden, Durant. Not only is Kyrie more talented but he is the superior fit next to Luka because of Kyrie's elite shooting ability and capacity to play both on and off the ball unlike Brunson who is mostly an on-ball threat (which makes it harder for him to produce at a high level alongside other high level players).