I expect the plan is for both to be in the Top 6 in 2025-26. Morrow needs some pro hockey experience although he'll probably get some callup time this season. Nikishin cannot play in the NHL before the last year of his deal with SKA runs out in the spring. There's been some fairly credible chatter that the Canes will sign him and bring him over as soon as SKA's season is done, which will let him click a year off of his ELC requirement. The right side of the D group in 2025-26 will probably be Nikishin, Chatfield and Morrow if everything works out the way it's set up right now.
And the D is only decimated if they have no plans to replace departing players, which is kind of silly. Of course they're refresh the group over the next two years, and not just with Morrow and Boom. Burns is noticeably slowing down (finally) and Orlov has been an awkward fit thus far, so it's not like they'd be losing those guys at their peak anyway. Carolina also really likes Joel Nystrom, who's coming over to North America for the first time at age 22.