While he's obviously a prodigy in the 400, his speed in that event will carry over to 800. I don't think he's done growing either, which will contribute to his eventual 800m WR.
He literally could jog a 49 split at this point. Honestly his build looks way more suited for 800. In the semifinal and the final it looked to me as he was outkicking most of the field he's built for the 800
He may well have elite 800 talent. However, if he wants a long career, the 400 is a much better bet if he has the talent for that. At least stick with the 400 as long as its viable.
800 runners have shorter shelf life, train way harder usually, and have endless supplies of Kenyans and others to contend with.
Letsrun never gets that sprinters aren't 800m runners. If it were so virtually every 400m record-holder would also be the 2-lap record-holder. They never are.
What Quincy's got going for him that's allowing him to be a prodigy, and will help him excel at the 400 or 800, is that he's ten times smarter than everyone else. His physical skills haven't entirely caught up yet, but he's 10th grade going on Ph.d. He'll figure out how to maximize what he's got.
This post was edited 14 minutes after it was posted.