I don't think it's possible to
prove that anyone will be anything, but we can discuss it if you like. I will even grant your criticisms: he looks funny and it's possible that he'll always look funny on the ice. My counter will be this: that doesn't mean he isn't effective.
If we're going to talk about 1C's we need to define what a 1C is. To me, it's a center who can play
a lot of minutes, whose line controls the majority of shots/scoring chances etc., and who is individually an offensively dangerous player. People will use zone starts to argue that Kotkaniemi is being sheltered, and I will argue that because hockey is a dynamic game, and coaches make line matchups adversarially. It's been shown that while players play better against worse players, and worse against better players, everyone is essentially playing against everyone else. Now, what about the effect?
What these mean, and what the data bears out, is that Kotkaniemi is ridiculously effective as a center iceman with respect to the control of scoring chances. That means that, as he matures, he's going to get
a lot of minutes. The biggest impediment to Galchenyuk becoming a 1C wasn't that he was awkward and jerky (and still is to some extent), but because he couldn't consistently get the minutes at that position. Jesperi has consistently expanded his role as an 18 year old, and no one has any questions that he will play a huge role and at center to boot. His own coach said that his play is forcing him to ''find more minutes.'' So, criteria one and two are fulfilled. Now, about criterion 3.
He can shoot. He might fan on shots, but he also has a diverse and dangerous shooting arsenal. He can shoot off of either foot, off the rush, on set plays and broken plays, from both sides of the ice, and he can beat goaltenders clean. He's going to score goals. As for his passing? ''Kotkaniemi passes'' became a meme before the season even began. Personally, I saw everything that I needed to see about his passing game in the first Liiga game of his that I watched. He understands passing angles in a way that few players do, and he's able to connect on passes that are unpredictable for opposing defences. That means that his wingers are going to score goals.
Now, if he never improves on what you have pointed out, then I agree, he will not be a 1C. But here's the (final) thing: he already has improved on those points. Maybe you didn't watch him last year, I don't know if you did, but he was
unbelievably awkward this time last year. In my wildest dreams I didn't think he'd have improved his skating as much as he already has. I'm betting that this will continue. Not everyone has to, but they should know what they're betting against.