Hill at RB was more of a necessity than being the actual plan. Miller not playing. Williams not playing and terrible when he did. You use what you got and besides Kamara, Hill was by far the most talented and productive.
Hill at RB was the actual plan. They talked about it in the offseason before and during camp which was before anyone knew that Miller was going to be hurt for most of the year. They had him at RB and FB during mini-camps, OTAs, and training camp. I don't blame Kubiak for doing it, he's a weapon playing those positions, but it didn't just happen by accident.
Still, he got hurt and you have to find ways to stay innovative and inventive without him which hasn't really happened outside of the scripted plays to being the game. I get that Kubiak is running a handicapped offense with lots of injuries, but there have been many games this season with 14 or less points, including against the terrible Giants Defense which seems pretty bad even considering the injuries.
I think Kubiak is an average OC and wouldn't hate it if he stayed, but I certainly would not be excited to see him return and I think there are better guys out there in the form of an offensive play calling HC if nothing else.