Now as for the Backstab ability I wouldn't want this to be taken from the rogue or the rogue/assassin due to the fact that one of their primary traits is stealth and so if you sneak behind someone with a dagger you should in fact get bonuses to hit and damage. After all, it's not every day that you expect someone to come up behind you and stab you in the back
That's not what's being referred to. You're describing Sneak Attack, which is in the game, and Rogues get it if they have advantage on the attack, or if there is a creature threatening the target.
What is being called "Backstab" is getting advantage by physically walking behind the target, who can't move or react to that because it is not their turn. Anyone can get it, without any ally assisting, even without being a Rogue. This is being complained about because it is an extremely low-effort way to get advantage and the enemy cannot react to it to prevent you getting the advantage if you do not leave its threatened area.
Ok Now that yo explained it that way I fully understand the only one who should get the backstab is the rogue and only while sneaking as it is in most games for which I've ever played like this.