Paying with a card is so smooth that it is invisible; paying with cash is so real that it hurts a little. That difference is the entire argument. Cash makes the cost present — you hand over notes and watch them leave. Researchers keep finding the same thing: people spend less when the money is tangible. You do not need a study to feel it.
The practical version is the weekly withdrawal. Decide the amount for meals out, coffee, and small purchases, take it out on Monday, and when it is gone, the week's small spending is done. No notifications, no surprise statements, no math. The envelope is the app; the balance is in your pocket.
None of this is an argument against cards — the points and the records are real. It is an argument for knowing what you actually spend on the small things, because that is the number nobody tracks. A month of cash will teach you the number. Then you can decide, with your eyes open, what to do with it.