But even that isn't so simple, because there are at least three types of people who use coupons.
The first are the ones coupons nominally target -- not people who go out of their way to find coupons. Just ordinary new customers who need an incentive to try the product once and then might become repeat customers. These are actually great when you can get them, but they're not that big a percentage of coupon users.
The second is the, let's say, analysts. Smart retirees who now have lots of free time and can use it to find a good deal. Tech savvy customers who know how to use advanced search features to find the best deal or are willing to set up a scraper to send them an alert. These customers are great too -- you don't make much margin from them, but you make a little, and then they immediately go away and leave you to keep their money in peace. And when you offer a good deal, they will come to you, so your margin is lower but so is your marketing expense.
The real problem is Karens. They don't know how to read but own a fanny pack that says "the customer is always right" and want to spend two hours arguing with you to try to get you to accept an expired coupon for the wrong product over something that has a $0.17 margin.