Yeah, I would assume Thomas will be packing his bags eventually. He's going to get chances though because I'm sure he shoots lights out in practice, but eventually it has to translate to the moments you get on the court.
Garbage time is not beyond worthless. It's worthless to look at Gobert stats in garbage time, sure. It's worthless to look at Niang's stats in garbage time (this season), but for guys who are at the end of the bench it's a great indicator of what kind of players they are (not necessarily stats, but just how they look/feel on the court). I remember Rayjon Tucker getting into the game and just chucking up the first bad look he got. There's guy who just look like they don't belong when they are on a NBA court and Shaq was one of those dudes. Sure, if you put him in a perfect situation and ask him to do very little he can succeed at not being a complete negative, but the moment you ask him to do anything on offense it falls apart, and when you run Utah's offense you can't have those kinds of players on the court.