I don't think anyone expects him to fail due to size, they just wonder if he can fill out enough to deal with the current demands of a top-line NHL center. I can't think of many bonafide franchise centers who are under 190. I think even people who are unsure about his ability to remain at the dot, expect a Patrick Kane type player. Using NHL.com top 20 centers list as a proxy for the elite centers in the league (its not a perfect list, but its something neutral to use in this example), the only guys under 190 are Barzal and Karlsson, who are both above 185. He's good enough to break the trend, but it also makes it not as easy to project his ability to stay there as a sure thing. Now, there are guys like Point and Trocheck playing at a high-level as centers who are under 185 (in Point's case under 170), so maybe the game is shifting in his direction at the center position. Giroux is the only guy recently with a smaller build who did it, and he played with a rare level of intensity, and Briere was also quite effective at center with that build.
If anyone's projecting him as anything less than a likely franchise talent, its most likely a hot-take. Barring a significant injury or the failure to develop anymore from what he is, its hard to see how he isn't at least what Clayton Keller and Marner entered the league as.