“Why do we have lines at all? Why not just whoever pushes hardest goes first?”
That’s how it works in the kugel line on Shabbos (which is also rude), but that doesn’t mean we have to behave that way in public…
I will sometimes let someone with only a couple things go ahead of me, esp. if I have a lot. If they were to then bring a whole cart full along, I would be annoyed.
As far as someone holding a place in line for someone who is shopping (and not just for one forgotten thing) – I think I would actually say, “I’m sorry, but the back of the line is back there.” What is so terrible about waiting your turn like everyone else?