And Kobach has the far more recent baggage of Brownback administration. Write off Johnson County if you want. Kobach can't win without taking Johnson and Sedgwick because he isn't taking Wyandotte and Douglas. He didn't last time around and I don't see how he does it in 2020.
For starters, I expect Trump will win by a wider margin in 2020 (at least in Kansas). Even if Kobach trails Trump, it is highly unlikely that the Republican nominee, whomever that may be, will lose a federal Senate race. That kind of ticket-splitting just doesn’t happen much anymore. Unless Gilligan-Sebelius gets in, they Dems don’t have a first-tier candidate to run, and KGS is no great shakes.
I’m not sure why you referenced Wyandotte, as it is the most Dem county in the state. Other than when Pat Roberts was effectively unopposed in 2002 (and the 2 nothingburger third-party candidates still got a whopping combined 43%), no Republican has won it with a Dem opponent since Bob Dole got his last term in 1992. Douglas lasted voted GOP for Senator in 2004 (when Brownback drew a desultory Dem opponent).
Ultimately, he doesn’t have to carry Johnson to win.