Of course not everyone is a superstar talent. That wasn't the point. The obvious point is the skill sets are similar enough that a CB often can move to safety. There are other less notable, but still successful examples, a couple of which I gave you earlier.
Consider this .... the Patriots have 9 players on their roster listed only as DB. They have a few listed as CB or safety, but overall the overlap of skills and duties is such that they don't even label many of them as exclusively a CB or safety.
We aren't talking about the difference between a TE who catches passes on offense and a LB who stuffs the run on defense the way you suggested before, this is two positions on the same side of the ball and in the same defensive backfield and that require very similar skill sets. Both positions have pass coverage responsibilities and run support responsibilities from the defensive backfield. Both need the speed and flexibility to stick with WRs. Sure they don't play an identical role in the defense, but the speed and instincts for a CB can very clearly translate to the speed and instincts for safety.
It's common for teams to shift college 4-3 DE's to 3-4 OLBs, or college OTs to OG, or college wide receivers to slot receivers. How do you think that is possible given that the positions aren't identical? The answer is because there is enough overlap in the skill sets and responsibilities that some may able to handle either, or they may even be better suited in the pros for the new position. The same applies to a CB, who if he has the speed and quickness to stick with a receiver step for step from the time the receiver releases from the line of scrimmage, should easily have the speed and quickness to stick with a receiver while giving him a 10-15 yard cushion.