The league also has walk-ons, as you state, not just draft choices.  Also, teams have free agents who are proven, in addition to the draft choices.

With the first round the only round with chances better than 50%, the drop off in success each round is drastic.  My bet is the Broncos aren't worse than any other average team out there in the draft.  We "fans" just tend to look at failure more than success.  And, again, it certainly doesn't help when the team keeps finishing well each year.  Getting that late first round pick isn't much better than a second round pick.