We see a lot of comments here at SamMobile from folks wondering why Samsung doesn't bring the Always On Display feature to the Galaxy S6 lineup or the Galaxy Note 5. Well, we'll get straight to the point here: Samsung probably isn't going to bring Always On Display to those devices because of their puny batteries. Always On Display is a battery-intensive feature (taking around 1 percent battery per hour) as it keeps some part of the screen lit up at all times, and it would be rather moronic to offer the feature on a device that can barely get through half a day without needing a charge.
The Galaxy Note 5 and Galaxy S6 edge+ do have larger batteries than the Galaxy S6 and S6 edge, and both phones also manage to last quite long for their battery capacities. Samsung could certainly bring Always On Display to these devices and leave it to the consumer to enable it (or at least add an option that enables the feature only when the phone is charging), but for the average Joe the decline in battery life would be rather frustrating. Putting in tiny batteries on the 2015 flagships was certainly one of the dumbest moves the Korean giant has made (removal of the microSD slot takes the crown in this regard), and whether we like it or not, Always On Display is a feature ill-suited to any of those devices.
But there is certainly a chance Samsung will bring Always On Display to the Galaxy A (2016) lineup, as it is offered on the 2017 iterations. Even the Galaxy Note 5 might get it with the Nougat update, but at this point in time, there is no way to confirm. It's best to keep expectations low; Always On Display is certainly a nice feature to have, but only when a smartphone is well equipped to handle the toll it takes on battery endurance.
