It would most certainly have been a different game with Niang in there, and ISU probably would have won. But, I think you have to give credit to UCONN with their perimeter defense, especially early on. That, and the fact that their bigs were able to bottle up Melvin. Now, with Niang in there, they may not have been able to do that because they would have had to devout someone to cover Niang, but would Dustin still have gone off for 34? Maybe, because I think a lot of his performance came from his own drive from being at home, and a lot of his family and friends there.
I think ISU could have won with Niang, but injuries are part of the game. Would ISU have won the Big 12 tourney if Embiid was not injured? We will never know.