This coupon to McDonald's is expired but I want to argue with the employee until they take it.
That seems like more trouble than it's worth. Why do you want to get McDonald's to take a coupon you know isn't valid?
Because I want to see if I keep pushing them if they'll agree to take it.
I know you want to get a discount on your meal, but it wrong to try to make a business honor an expired coupon. Arguing with the employees is rude and inconsiderate. If the coupon is expired you should not try to use it.
I don't see why it's so wrong.
It's wrong because by doing so you're really inconveniencing the employee. They aren't really supposed to honor the coupon, but they also don't want to get into an altercation with you, so they'll be forced to take it. I don't think it's worth the trouble.