I know some will object to this, but maybe Parcells just isn't the coach he used to be.
I think it's more the case that Parcells was never the coach people think he was. He just happened to have an actual genius coaching his defense along the way and benefitted from that. Parcells 'on his own' is basically Wayne Fontes, and thus it's not shocking that the results and chaos at the end are Fontes like as well...Look at the records:
Parcells with Belichick (e.g. what people thought Parcells was): 117-73 (.616 W%), 11-5 playoffs, 2 Super Bowl wins
Belichick without Parcells: 104-78 (.571 W%), 11-2 playoffs, 3 Super Bowls
So, Belichick didn't miss Parcells at all.
Compared to to this:
Parcells without Belichick: 49-53 (.480%), 0-2 playoffs
Fontes: 66-67 (.496 W%), 1-4 playoffs
To me, as he would say, Parcells is exactly as good as his record says he is. His 'alone' record, that is.