Passengers going through without document checks is a minor element of Schengen. The heavy hitter are the trucks passing borders much faster and with no admin expenses - saves EU countries many billions of EUR annually.
In a pure economic efficiency sense you might very well be right. But the "illusion" that borders without border control used to give shouldn't be completely disregarded either. Pre 2015 (and Corona) when you could just roam around without ever showing ID just felt very different. It didn't feel like I was moving between distinct and sovereign countries, but more like moving between states in the U.S: "Oh now we crossed in to Texas cowboy noises".
Especially if you live in a border region (e.g Malmö-Copenhagen) and commute across the border, you really feel the difference and might even consider stop doing that because of hassle and increases in travel time.
During corona the closed Finish/Swedish border in Happaranda/Torneå just felt surreal, most warlike experience I've ever had. These cities are essentially one, and all of a sudden families, friends, businesses and their employees were split with fences and armed people stopping them.