Also, the OSU AD probably wanted to be able to show he's retaining a nice chunk of the remaining "successful" staff as for some people this would help to combat how poorly he handled retaining Underwood - from an OSU fan perspective. Whether the assistants will be successful without Underwood, as well as whether Underwood will be successful without those particular assistants will remain to be seen for a couple years. I honestly hope both work out well.
OSU lost Underwood and opted for the inexperienced assistant. From a financial standpoint, they saved a ton. From a fan and PR perspective, they went cheap. So offering an attractive package to the other assistant coach to soften the blow was kind of expected.