Oh I don't think anyone is saying 100% that Russell is replacing McNabb (especially since it sounds like he's not quite fit), but the #5 experiment in Washington is certainly already in its twilight, regardless of who it is.
There is nothing in McNabb's resume that points to him being a guaranteed Hall of Famer. He has always been woefully inaccurate (balls thrown into the ground account for a vast majority of his incomplete passes). His fear of throwing interceptions also accounted for a number of balls thrown away, instead of attempting to thread the ball to his receiver. Good quarterbacks in history aren't afraid of their stats all the time and will attempt the
hard pass in order to advance the team.
The fact that he didn't understand simple rules about the game he played (Overtime anyone?) meant he never grew into the leadership role that a quarterback needs to play. Most of his former teammates found him goofy and childish, although anything Freddie Mitchell said about him I'm willing to dismiss

.
So we have a guy who couldn't run a 2-minute drill properly, couldn't lead from his position, could never make the risky play or even attempt it, was horribly inconsistent with his throws his entire career, and never pulled off the big win. Not what I'd call HoF material.