For me, defeat against the Steelers should have made McDermott's seat very hot.
I always said a game against the Chiefs is a 50/50 affair and, to date, McDermott/Josh have had the upper hand in the regular season whilst Reid/Mahomes have got it done in the play offs. Whatever happens against them, he won't get moved on.
Do I have my doubts about having a defensively minded HC presiding over an elite QB? Yes. Less because of McDermott himself but more because of the lack of consistency on the offensive coaching side. If the Bills win the SB with the offense playing at least an equal part then some teams may have a look at Brady, and so the cycle will continue.
But will McDermott be sacked with a blowout or infamous ending against the Chiefs? No. And what he's done from Dorsey's sacking onwards has been remarkable.
I can see Rivera joining as a consultant but not as DC. I do think McDermott struggled early on holding both the HC and DC roles but he seems to have adjusted as the season went on. If he brings someone in, it'll be a coordinator who will follow his style of defense, and won't raise too many questions or disputes.