The 2015 season was actually something of a bounceback year for offense. But that doesn't mean that the corner has been turned in what has become an era of increasing pitching dominance.
The standard measurements offer clear evidence: 2015 was a better year for hitters than '14. Runs per game increased from 4.07 to 4.25. Combined ERA increased from 3.74 to 3.96. Home runs increased from 0.86 per game to 1.01.
Marginal gains were made by the hitters in 2015. These numbers seem to be at least superficially encouraging for increased offense. Looking ahead, could this be the start of a positive trend for hitters?

Stats according to Baseball Reference
More likely it was simply a relative measurement, the result of the fact that 2014 was an even better season for pitchers. The problem with reading too much into the small gains of 2015 is that '14 was a historically difficult campaign for offense.
That average of 4.07 runs per game for an individual team was the lowest in a full season in 38 years, since the 3.99 in 1976.
The 3.74 ERA was the lowest in a full season since the 3.71 in 1989. The 0.86 home runs per game was the lowest since 0.72 in '92.
The averages over the past 20 years show the gradual shift from offense to pitching. Those changes reflect the institution of Major League Baseball's rigorous program that has severely limited the use of performance-enhancing drugs. But those changes also reflect a greater emphasis on the organizational development of pitching, in greater quality and quantity.
The current numbers, while considerably reduced from the late 1990s and early 2000s, are still nothing like those from the ultimate Year of the Pitcher in modern baseball, 1968. In that season, teams scored just 3.42 runs per game, and the overall ERA was 2.98. Teams hit just 0.61 home runs per game.
One clear measurement of pitching domination, or a change in hitting philosophy, or both, can be found in strikeouts per game. In that area, 2015 was not a better year for hitters. The mark of 7.71 strikeouts per game was just barely above the 7.70 recorded in 2014. But it was also the highest mark in the history of the game. And it marked the 10th straight season in which strikeouts per game had increased.
That is no accident. It is no surprise that offense is generally on the decline when the two-strike approach has in many cases vanished. Where is the short-stroke, contact-making approach that Cubs manager Joe Maddon refers to as "the B-hack?" Apparently, the stigma attached to striking out has also been diminished. The strikeout is the same as any other out? Not with a runner on third and less than two outs.
Maybe the corner has been turned and the hitters are poised to make an across-the-board comeback. The numbers from 2015 might suggest that this could happen. But three or four straight seasons of the same trend would be a lot more convincing.