Actually, Sirianni isn't a bad coach. Is he HC material? Who knows. One thing going for him is he understands QBs. Put a decent staff around him and he might work out. He probably wouldn't be my first choice, but considering their problem at QB it makes sense. Philip Rivers liked him a lot and IMO Rivers would be a very good OC if given the opportunity. Both Rivers and Sirianni would talk X's & O's with each other.
Rivers many times talked about wanting to be a coach. It would be interesting to see if Sirianni brings him in as a QB coach. Many people may not like Rivers but in his prime he was as good as anyone and he had what might be a HOF worthy career.