The 2015 NFL season was the seventh straight year for an increase in the number of defensive snaps in the NFL on which five or more defensive back were on the field. Per Pro Football Focus, here is the percentage of plays in NFL games with five DBs or more on the field in every season since 2008:

2008: 43.4%
2009: 45.1%
2010: 48.8%
2011: 52.5%
2012: 54.4%
2013: 58.3%
2014: 60.3%
2015: 63.4%

The NFL, as the Pro Football Focus numbers show, has transitioned into more of a three-down passing league.