My opinion....
Kubiak will struggle as HC next year. He had some pretty noticeable struggles and growth as OC this year. Taking on the entire organization is a large change. McDonald could be a great mentor on how to do that, but it probably takes them both another year or 2 to have seen enough and lived through it to translate to a new environment. So, if that's true, does the increased risk of falling overcome the reward of a big raise and a lifetime dream job?
From the Seahawks perspective, Kubiak seems replaceable...to me. He's done much better than Grubb, but he's also had games where he seemed to lose track of the foundation and got caught up in the game flow. ...more early in the season. It seems to me that the Macdonald hire and Kubiak bounce back hire shows the organization can, if needed, land a Coordinator that fits and still make a deep playoff run in year 1.
If Seattle wins the SB, it's reasonable to think a lot of teams will include Kubiak on their candidate list. Interviewing him now is just a timing thing.