When you go 4-9, 7-6, 7-6 with some bad losses, changes have to be made
In most higher profile programs, significant changes have to happen at that point, just for the HC to show the AD and fans that he understands it is not acceptable to them or him. The OC or DC would be let go by that point, or maybe both. Already replaced the OC. So bringing in a new, experienced DC would be expected by most coaches at this point. Not sending a bad message to anyone, except that there is accountability.
I am not so sure that BYU would not pay what it takes for a good DC these days. I doubt Grimes came cheap.