Why does the RBF function still remain in use?

The ability to toggle the RBF function by manipulating the sequence of transaction inputs is completely meaningless. Even after disabling RBF and broadcasting the transaction, there is still no obstacle to replacing that transaction with one using a higher fee. In short, there is no difference whether you enable or disable RBF before broadcasting. On the contrary, the fact that this feature still exists creates a serious side effect by misleading people into thinking that a transaction is irreplaceable when sent with RBF disabled. Does anyone know why this hasn't been removed?