The point is you had no obligation to do anything but tell the FA that there was a bag under your feet, but it makes no sense that the FA wouldn’t ask the passenger in front of you if it was his bag.
And I don’t know where you get “acceptable”, nobody said you should leave the bag at your feet.
Another passive-aggressive way of handling this would be to step all over the bag, spill your water on it, and generally make sure anything in it was ruined.
I mean, there’s a whole range of passive-aggressive behaviors that could be applied to this “problem”.
I had no obligations. I was not even obligated to tell the FA. The passenger however was obligated to put his bag where it belonged. His violation of his obligation and outright selfishness in willfully optimizing his own space at my expense merited no consideration in even the leasest amount.
Never yield to bullies and always fight back 2X harder. It is the only thing that works.