I disagree that it's a given one of Jokipakka or Oleksiak are gone. I think that's an option, but I think it's just as easy both remain. Dallas only will be forced to move one defender, and I'd say Daley followed by Goligoski are moved first before the young guys. Again, that's an option not a given. Moving only one player gives you 7 NHL D and no need to make a change.
Personally ... my guess is Daley or Goligoski are moved and then they do their best to acquire a two-way D with a leaning towards defense or at minimum a defensive defender. If they find one they like, whoever is left after the first move to get to 7 would still be an option to move this time ... so the remainder of Daley, Goligoski, Jokipakka, or Oleksiak. Personally I add Benn to that group as well. I don't think it's unreasonable to think you could start next season with both Oleksiak and Jokipakka, one of the veterans in Daley or Goligoski, a new defender, and Benn out.
They won't take moving Oleksiak lightly. His potential is the exact defender they are said to be looking for: a big bodied D for the defensive zone and the added bonus of some potential offense. I'm not sold that he's a lock to reach that potential, and I don't doubt they have some concerns as well, but they don't have any other option to fill that role remotely close to the NHL. All of their closest players are high end offensive talents in Honka, Lindell, or Backman or smaller, mobile two-way options in Bystrom.
I'd argue the only change Oleksiak moves this summer is for a similar, already developed player. Otherwise, they trust their development. Jamie looked much better in the NHL than he did the AHL. I don't think that's necessarily a good thing because i think he has an attitude problem about going down. That might just be an immaturity issue or it could be more. Still, that's no reason to take away your only other player that could offer you some size like Nemeth.