Hindsight is always 20/20, but I would say that the last two staffs, (Frost and Riley) made mistakes right off the bat by not keeping anyone from the previous staffs. I know Riley kept Warren on, but he left right after signing day and didn't coach a game for him. For Riley, the only one to me that would have made sense was probably Garrison. Young energetic OL coach who was a really good recruiter.
For Frost he maybe had 3 options, Parrella, Bray or Williams. Of those three I would have probably gone with Williams for his coaching ability and recruiting prowess.
I can see why both coaches didn't, but with the "buy-in" issue being talked about this year and I am sure it was an issue for Riley, a coach from the previous staff can help with that, especially in the first year when things get rough.