If the primary winner accepts the appointment, she will be trading a sure two years in the Senate for a chance at eight years but with all the fundraising and campaigning that goes with it. She will have to run again as an incumbent in 2020 to finish the term, and again in 2022 for the chance at a full six years.
Since this will create a vacancy in the general election, the state Republican party will be free to select any replacement candidate that they want. While the runner-up in the primary would be nice, it doesn't have to be that person. However, it will need to be somebody with a ground organization in place, so a primary candidate would make sense.
If the primary winner would rather have the direct shot at an outright six-year term, then the voters might not see the runner-up being appointed as a slap at the winner, because that person will need to run again two more times.
-PJ
The idea was suggested to me by an AZ person that Ducey may want the seat in 2020 and therefore wants a “temp” in it.