I don't disagree with you.
Historically, I've preferred to bring in experienced people who have demonstrated sustained success at our level.
That said, it's not black and white, and a track record of success doesn't guarantee success.
Look at Tom Herman, for example. The guy goes 22-4 at Houston and then 32-18 at Texas. He should have been a guaranteed success at his next stop, right? Instead, he goes 6-16 at FAU, which has been a respectably competitive program in revenue sports over the last handful of years. What happened?
Hugh Freeze went 20-5 at Lambuth, followed by 10-2 at Arkansas State. He should have been a success at his next stop, but instead, he went 12-25 at Ole Miss. People thought he was overrated and a fraud. He turned around and went 34-15 at Liberty before being poached by Auburn, where he's now 11-14. What happened?
Scott Frost went 19-7 at UCF and then completely flopped as head coach at his own alma mater.
Gus Malzahn went 9-3 at Arkansas State followed by 68-35 at Auburn. The guy should have been able to blow out anyone in a non-SEC conference, right? Instead, he barely squeeks out an above .500 record at UCF.
All this to say, for every "successful and proven" coach that flops at their next stop there's an up-and-coming coach who absolutely thrives in their next stop at an elevated position (coordinator, HC, etc.). There are so many factors that contribute to a coach's success at any given stop, so predicting success solely on past performance isn't a perfect science.
The truth of the matter for every coach at any position is that you just don't know how they'll perform until you know.
Until we know for sure, let's give Cassity the benefit of the doubt and the support he deserves to succeed at UNT.