Like him or not he has a MASSIVE following of people under theater, both musical theater fanatics and not. People who probably do not know this show or have heard of it and never got around to diving into the score on the beautiful original cast album, and have never seen it live. I have no doubt they'll try to make a cast album because of Platt, and I hope they do.
I think that while he seems too young for this role right now, he is 2-3 years younger than Leo Frank would have been when the play takes place... he is also haunted by the perception that he looks older even than he is. I'm more worried about the "youthful" nature of how he comes off when he acts and sings... getting the adultness of Leo convincingly and naturally I think will be an acting challenge for Platt. But he is a very good actor, and has a beautiful voice so this will definitely be a gorgeously sung production either way.