Agree for the most part but to a slightly lesser extent.
Shanny definitely is not a good coach, but he isn't a "walking disaster". A walking disaster at HC is Mike Singletary, Petrino, or Cam Cameron. Shanny is just consistently average. Never has very impressive or competitive teams, never a playoff threat. Usually his teams are .500, occasionally a little worse, occasionally a little better. Like you said, it is not a coincidence that he hasn't done squat since Elway left....if you're a good HC, you win more than 1 playoff game in that (massive!) time span. Period.
IMO, Shanny is no different than Jeff Fisher. Coaches who have been in the league a long time but consistently field average teams.
Also agree than RG3 has an uphill battle. So does any early pick at QB tho. Being taken that early does, after all, imply you are going to a team that was nothing short of a disaster the previous season. Shanny's poor record of finding and developing talent at the position also does not bode well for RG3. With that said, he is an awesome prospect with lots of positive qualities so it should also be no surprise if he is a total stud.