The passenger was petty, a petty tyrant and bully. So taking someone else’s leg room and feet space is acceptable so long as no one complains? No I raised the stakes on such behavior. That person will think again before they try to take what isn’t theirs.
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”.