It's mostly true that people will substitute one purchase for another. They have x dollars to spend and in the end spend x.
For example, movies are $15 but on Wednesday night they are only $10, so the viewers go on Wednesday and spend the $5 saved on popcorn. Did the movie theater make as much money? No. The movie company's expense to show the movie was the same, but they had to pay to staff the concession stand and for the popcorn. More expenses versus the same revenue means less profit.