Two things to consider about DJ. One, he is already 21 and has been in college for three years. True, this was the first year he got serious minutes, but he might not get the same "potential future" bump that he might've if here were even a year younger and certainly not if he had this year as a freshman.
Second, saying that NBA draft is all about futures is sort of true, but potentially misleading. When teams project a player's future in a draft, they have to gauge what the level of potential is - for example, superstar, all-star, starter, 6th man, etc. -- and how likely he is to reach those various levels. My guess is that teams are not drafting DJ with the thought that he has a decent shot to be an all-star. They're projecting him to be a potentially valuable floor-spacer with defensive versatility and upside. A year in college could allow DJ to show that there is a very high chance he can fill that role or even that he does have potential to be at least a second options kind of guy. In other words, he's probably not a lottery guy now and a year may make it significantly more likely that he gets picked in the first round.
However, a lot may depend for him on how he shows at the combine. If he can demonstrate high athleticism, then it really may be best for him to go now. And a lot also depends on the feedback he gets. Maybe I'm underselling what NBA teams are seeing in him as far as potential.