I also think part of the change in the secondary is in the current offensive approach to the game and the skill-sets of many of today's offensive players. While the health (injury) situation at safety may have something to do with the interchangeability of cornerbacks and safeties for next year's team, there has also been a trend, as a response to the increasing complexity of offenses in football, to change up the approaches and looks in the secondary, to give the opposing offense something different to look at. It is a little harder for the offense to attack a specific position in the secondary, such as exploiting a faster receiver against a safety in man coverage in a three/four wide receiver set, if the DBs, whether there are 4 or 5 of them, are employed in a more interchangeable manner. Just as offenses are blurring the traditional distinctions about positions, becoming more flexible, so have defenses begun to respond.
As today's football players, especially at the offensive "skill" positions, have become more athletic and versatile, the traditional distinctions concerning defensive positions have been adjusted to respond.