They don't have to worry about paying Duren for another 15 months. Unless he takes a team friendly deal like Stew did there is zero reason to sign him this year.
From Duren's side, there is also no reason for him to sign this summer. If he thinks he is an up and comer he would be better off putting another season, and another playoff run, on his resume. This summer, he can't negotiate with other teams. So he has no external pressure to put on Detroit to give him a larger deal. Wait until summer of 2026 and actually enter free agency so he can negotiate with everyone. It is also a decent FA class that summer, so teams are starting to maneuver to open up cap space. Another plus for a guy like Duren.
If he stinks it up in this years playoffs, you have to give considerable thought to trading him this summer. If you feel Ausar is part of this team's future, I don't see how Duren also is. You can't play two guys at once that are not a threat from more than 5 feet from the basket.
The picture will be a bit more clear after the playoffs.