To be fair, only two teams have won a Stanley Cup in the cap era without a dman of that calibre (Carolina 2006 and Pittsburgh 2017). The rest have all been Hedman X2, Keith X3, Doughty X2, Pietrangelo, Chara, Lidstrom, Pronger/Niedermayer, Carlson, Letang, and Gonchar.
So it's a borderline requirement to have a dman of that calibre to win the Cup. My definition of "#1 dman" is that you can realistically see the player fill that role on a Stanley Cup winning team, and not have it be a major anomaly like the Carolina/Pittsburgh wins. So Rielly IMO is not a #1 dman - he can fill the role on a pretty solid team, but to be a true top contender, you probably feel like you want an upgrade there.