Nill does seem to look at things long term. So, he seemed to have decided that Kevin had little more upside than what he had shown, whereas Johns and Lindell (and later Honka) still have upside.
When he knows/strongly thinks he is right about a player, he signs them, a la Klingerg and Johns. If he doesn't like a players future, he gives them the one year deal, a la Campbell, Oleksiak, and Dillon, and they are let go or traded for something useful if possible (I think the Demers swap was a bigger thievery than the Seguin deal, really)
It looks to me like when he realized there were no Hedman type D available in trade (and why would there be?) he was going to grow his own D core, and Johns was a bigger part of that deal than Sharp. We took Sharp's contact because we could, and we really wanted Johns. Given Nill's talent evaluation skills, I am not sure John's won't be elite, or very near it. If that's true, and the Stars win the Cup any time soon, then dealing two near zero value parts you wanted to be rid of (and so did Chicago when they got them, although both are doing better now) for a short term scorer and long term shut down D will be looked back as one of the key moves in Nill building a winner.