Crosshairs are unrealistic because they arent really part of the game. They do not exist in the game world, instead they are just coloured markers on your screen. Crosshairs are neither realistic nor unrealistic. However, a monocle computer in the 80s, that is highly unrealistic. The HUD is okay to have so long as it does not become some thing like the GRAW monocle computer which is part of the gameworld.

I rather enjoy not having a crosshair in TC:E.
Adding a crosshair is just going to be copying every single generic fps known to man.
If you don't have a crosshair, however, it forces the player to think smarter (because firing blindly without a crosshair rarely ever gives a positive result at anything but super-close range).

smarter players = more interesting firefights

that's what a tactical-realism shooter is all about, keeping encounters raw, gritty, and memorable without players relying on dumb luck as their primary strategy.