If it were the case that this film were made as an original film musical without the show as a precedent, one thing that likely would be different is that it would not be starring Ben Platt. Even if Platt had achieved fame in some other way (since his starring turn on stage would not have existed), I doubt that the powers that be, even if one of them was still going to be Platt's father, would have had any real reason to consider a nearly 30 year old actor to play this role.
Would that make a real difference to the movie though? I don't know. One of the attractions of this film is that it preserves an iconic stage performance, something that we all complain doesn't happen enough. Without that carrot, does the film sink deeper into a plotline that many now find distasteful or does a charming, stellar performance from another (teenage) actor elevate the film without all the weirdness surrounding Platt's age.