Good point, Cernak?
Cernak is possible, but he again won't cost THAT much to re-sign and he's also a right shot. They have Hedman, McDonagh and Sergachev who are all left shots and combined make 19.425M
Moving one of them sheds more cap, which they need to do, and probably gets them a not too bad return. Sergachev is overrated and less important to them continuing their dynasty than McDonagh is. He also could bring them back a 1st+.
My guess is Sergachev for a 1st + prospect (no cap coming back) then they pay Seattle something like a 2nd + Joseph to take Johnson. In that situation they'd have managed to shed 9.8M cap, putting them 6.2M under the cap.
They'd still have all of their meaningful forwards (Kuch, Stamkos, Point, Cirelli, Gourde, Killorn, Palat, Maroon, Ross) and a solid defense (Hedman, McDonagh, Cernak, Rutta, Foote) and be 6.2M under the cap while only needing to sign 1F, 1D and a backup G. Still would be in a strong position for a potential 3peat.