For the longest time, games with the Battlefield logo slapped on the box art were often considered to be the most realistic FPS multiplayer games that you could buy. While this was almost certainly an exaggeration, EA did little to dispel the perception. If anything, EA encouraged people to think that this was the case, especially over the course of the last several years. Marketing buzzwords and robotic arms on female British commandos aside, EA was actually telling the truth (for once), from a certain point of view.
Compared to its competitors, the Battlefield franchise is quite realistic. You can't run across the average Battlefield map in half a second, the environment can be deformed by explosives, bullet drop is a fairly unique mechanic that separates skilled marksmen from amateurs, and suppression is a relatively new mechanic that attempts to simulate the psychological effects of being shot at. If you were to compare Battlefield 1 with Call of Duty: WWII, the former would be far closer to being a war simulator than the latter, which is often (rightfully) regarded as an arcade shooter. When you factor in DICE's indisputably excellent work when it comes to Battlefield's audio and graphical effects, it shouldn't be very surprising to hear people say that Battlefield delivers an authentic and immersive experience.
However, such comparisons fall flat when you realize that EA's intended audience for Battlefield is the casual, mass-market gamer. In other words, EA isn't trying to make Battlefield more realistic than it needs to be, and it only needs to be as realistic as the average gamer is willing to stomach. While the realism of some of Battlefield's more fanciful mechanics can be dismissed for the sake of gameplay and maintaining an appropriate ESRB rating, it doesn't take a history professor or an active duty serviceman to tell you that the average Battlefield game is not substantially more realistic than the average Hollywood war movie. A quick look at Battlefield V's debut trailer is enough to give a good sense of this, and Battlefield 1 is certainly guilty of warping reality (especially in regards to weaponry) for the sake of good gameplay. Naturally, DICE can make Battlefield V more realistic, the question is whether or not they can learn from other games that aim more for realism than fun for the masses.
For example, games like Red Orchestra completely do away with the traditional UI—you are responsible for keeping track of your ammo and reloading at the appropriate time. In addition, virtually every weapon will do enough damage to kill with a single shot to the torso, though your character isn't 100% accurate with every weapon in existence. Getting shot in the arm or leg predictably hampers your ability to fight. As far as vehicular combat goes, tanks cannot be crewed effectively by a single person. In games like Brothers in Arms, suppression is a primary feature of the game rather than a side feature that exists purely to inconvenience snipers and people who love to just run into a hail of gunfire. If modern conflicts are more your forte, the ARMA series is notorious for being realistic to the point of near inaccessibility, though it did spawn the incredibly popular DayZ mod thanks in part to its commitment to realistic gameplay mechanics.
Even if you were to disregard the aforementioned games' mechanics, they all share a thematic commitment to realism. The uniforms, equipment, and the soldiers themselves all presumably resemble their real life counterparts quite closely, or at the very least they don't immediately stand out like a sore robotic thumb. Weapon variety takes a significant hit as a result, but if you want realism in your games, then you can't expect to see entire squads of soldier equipped with completely foreign weapons. DICE can certainly borrow elements from more realistic games for the sake of enhanced authenticity in Battlefield, but at that point, it brings up the question of whether or not it would still feel like Battlefield.
It is understandable that hardcore Battlefield fans would feel disappointed, perhaps even betrayed after watching the Battlefield V debut trailer because it shows off some incredibly unrealistic and inauthentic moments in a franchise that is marketed as being realistic enough for the masses, but at the end of the day, DICE is evidently trading realism for the sake of fun. Well, fun and profit. It doesn't excuse the inauthentic feeling of the trailer at all, but it does make the dramatic shift in tone comprehensible.