Of course, there are exceptions, but how many of those "thousands" transferred from a top tier D1 school? Not the 2 named above.
The point that you can transfer from a top-tier school, tear up at the non-AQ or FCS level and make it to the league still holds. Clearly these are exceptions, but it is a possibility, given the right circumstances.
From the last ten NFL drafts:
-Rhett Bomar went from OU to Sam Houston State before being a 5th round pick in '09 (obviously not a regular transfer, but his story still works anecdotally).
-Joe Flacco was beaten out by Tyler Palko at Pitt before going to Delaware and becoming a 1st round pick in '08.
-Colt Brennan was a walk-on at Colorado before transferring to Hawaii, killing the WAC for two years, and becoming a 6th round pick in '08.
-Ingle Martin (maybe the best comparison on this list to what Derby could look like if Rudock beats him out) started a few games at Florida before getting beaten out by Chris Leak. After transferring to Furman, he was drafted in the 5th round in '06.
-JP Losman was beaten out the starting job at UCLA before transferring to Tulane and being a 1st round pick in '04.
-Andy Hall transferred from Georgia Tech to Delaware before being picked in the 6th round in '04.
-Josh McCown oddly left SMU after throwing for 4,000+ yards in three years and QBed Sam Houston State for a year before being drafted in the 3rd round in '02 (like Bomar, this isn't a perfect example.
-After losing the starting job to Rohan Davey at LSU, Craig Nall transferred for a year to Northwestern State (yes, that Northwestern State) and was a 5th round pick in '02.
-Wes Pate started his career at Louisiana Tech before transferring to Stephen F. Austin and getting drafted in 2002's 7th round.
So there are a bunch of guys that have transferred down a level for one reason or another and gone on to become NFL draft talents. Does this mean that Derby will transfer? No. Does it mean if he did he would play, start, star, or get drafted? No. But there is a precedent there that is too significant to ignore.