According to some Steam stats it appears as if DayZ has dropped from 23,000 average players in January, 2014 all the way down to 6,000 average players as of June. It's safe to say that the drop has been rather significant and the game has a lost a ton of its player-base over the course of just a year.
The stats were posted on /r/Games/ where it links to the DayZ Steam Charts, showing the average users, the gains and the peak users. DayZ saw its biggest peak when it launched in December of 2013 to the tune of 45,000 users logging in to play the game. It maintained strong numbers throughout the first half of 2014 and it even had sales to match, with millions copies being sold within so many months of its release. In fact, it managed to hit 2 million copies sold from December to May.
However, after July of 2014 the decline became more steady and precipitous as throughout the early fall. Things quickly picked back up during the late fall and early winter of 2014, mostly likely due to the Steam winter sale, but then things began to decline once more. Throughout 2015 there's been a steady drop in the player-base with the peak in March being 29,000 while the peak in May dropped all the way down to 13,000.
The conversation on Reddit is interesting because there are claims that Bohemia Interactive really messed up by having the game head into Early Access at the end of December, and that they should have moved to a different engine. Others said that they shouldn't have entered Early Access at all.
However, if we really take a look at the progress of the game and the time-frame of its Early Access release, we realize that the game came out right during the peak of its popularity and Bohemia Interactive and Dean “Rocket” Hall managed to cash in at a perfect time.
There was a lot of competition in the zombie survival genre and had they waited they may have missed a prime opportunity to make headway when they did. At this point in the gaming scene the big thing is dinosaur survival, with games like ARK: Survival Evolved taking over the gaming sphere.
As for the game engine... DayZ is now running on the new Enfusion game engine, which is an upgraded version of the Arma 2 engine. Some felt that the team should have just skipped to the Arma 3 engine but more savvy techies brought up the very pertinent point that the Arma 3 engine still suffers from a lot of drawbacks and doesn't provide any solutions for what the DayZ team had to deal with in order to get the game to be as playable as it is.
I don't really know what Bohemia Interactive could have done differently, it just seems like at the moment the nature of the beast is taking effect. Having an Early Access go on for more than a year would definitely diminish the player-base for an online-based game like DayZ, but waiting to launch the Early Access at a later date would have given the competition a leg up and they would have lost serious market share and mind share.
As it stands, DayZ is still trucking along and is scheduled to hit completion sometime in 2016.