So yeah, you can get advantage pretty easily but so can your opponents, it still comes down to better positioning and when all things are even usage of resources.
Still you fail to see the impact of dishing advantage in the overall weight of some classes that are highly dependent of it as Barbarians with reckless attack. Advantage is gold in DnD5e. Advantage in BG3 is a must. See the difference?
The class feature, if implemented as is, would have relatively diminished returns just like rogues see diminishing returns on having bonus actions. You still have a net gain (but by how much) for having another source of advantage for when you can't flank for the free backstab or for when there's a height difference that cannot be overcome.
You're correct in the weight of classes being shifted, but that primarily depends on how the class is implemented and how much in the grand scheme of things. Having the weight shifted is meaningless without taking a look at the context as a whole. The 5e system is not that fragile.
The only case u cannot backstab is when there are already 3-4 characters stand behind the enemy, which is very rare. Als your argument of diminishing return is not very convincing. Additional bonus action is very powerful even in high level.
