Well, you've got some valid points. However, my arguments are based on what Broncos are thinking. Their confidence on Foxworth is not as high as you, because, they've been actively searching for CB around the league after Williams' passing.
I disagree with your assessment on Bly being average DB. He led the Lions with 6 Ints, despite missing four games. Yet, he made a viable enough impact to be selected as a first alternate for the 2006 Pro Bowl. He's played for a poor team with awful offense, which allows defense to stay in the field longer, not to mention that he had to go against the top receiver every time.
According to Stats, Inc., Bly has the most forced fumbles (17) among all cornerbacks since 1999 (second among all DBs). He also ranks sixth among DBs (fifth among CBs) with 113 passes defenses since he was a rookie in '99, has the fourth-most interceptions (33) and ranks fourth among DBs in defensive touchdowns with eight (48 points) during this time.
All that leads me to believe he'll succeed in our uniform.