I am thinking his floor is probably a #3/4 starter which would be a guy that averages 9+ k a game and walks probably aright around 4-4.5 guys per nine.
How many qualified starters last year had a walk rate over 4? 0. How many qualified starters in 2016? 5 and 2 of those were vets who'd proven themselves better prior(Shields/Liriano), 1 was Jimmy Nelson who just had a down year and the other two were Brandon Finnegan who's probably going to the bullpen soon or later and the last guy is Tom Koehler who's basically a guy Miami had to use as they had no other options for their #5. In 2015 there was 1 guy in Trevor Bauer who had the worst walk rate of his career. If you want to be hyped about Koepch fine. But the "floor" you're putting on him isn't a #3/4 starter. It's not even a starter save for a few rare cases. Your aim instead should be basically a right handed Robbie Ray(10.69 k/9 3.79 bb/9) if you think he's a starter. But it's a pretty big assumption to just assume he's going to drop his 4.8 bb/9 in the minors to 3.8.
As for his stuff, not sure where you're getting that he has an above average change up(currently anyways) from. Everything I've read on him suggests his change up is inconsistent. As I said else where, it's entirely plausible that he develops it enough that it becomes consistent but he doesn't currently possess 3 average or above pitches and you're not a starter with 2 great pitches.
Frankly I don't really care one way or another what his top end is. The only reason I took issue with your comment is it's completely ignorant to suggest he had 0 chance of being a closer. If you want to say there's a 75% chance he's a starter fine but suggesting it's guaranteed is just being a homer. Hell, even if he had none of the issues I mentioned above, guys who throw 100 don't stay healthy. And again, if you want to believe he's the exception to the rule that's fine. But at least acknowledge the possibility injuries could limit him to a bullpen role.
And as I said before, I really don't even see why you're scoffing at the idea. Elite closers are incredibly valuable. The cubs gave up Gleyber Torres for 3 months of Chapman. They gave up multiple years of Soler for a year of Wade Davis. Obviously you give him ever opportunity you can to be a #1 or #2 starter but if the issues don't iron out and he's a worse Drew Pomeranz(8.79 k/9 3.77 bb/9), I'd honestly rather have someone who could be Kenley Jansen, Craig Kimbrel or Chapman closing out games.
Ultimately I'll leave this topic where I started, with Carl Edwards. Edwards in the minors had 11.5 k/9 and 4.3 bb/9. Kopech's at 11.8 k/9 and 4.8 bb/9. If you want to say Kopech has better stuff I'll concede the point but ultimately that's not why Edwards ended up in the bullpen. He ended up in the bullpen because of the walk rate and the fact his change up wasn't great. If you're unwilling to see that possibility with Kopech you're being willfully blind and there's not much worth discussing.