Those detergents do work. You don't need to use them all the time, though. You can just use them occasionally and it will breakdown the buildup from the times when you used regular gas.
I am curious to what the buildup you are referring to is, fuel injectors maybe? This might make sense on older multi-point fuel injection cars where their max power is like 60 psi, but on modern direct injection engines the fuel rail pressure is so unbelievably high that the sheer pressure would slice through any form of buildup. (My VAG engine for example, idles at ~700 psi and easily reaches twice that when moderate acceleration is demanded)
Ethanol is fantastic at removing deposits. So much so when it was phased in 20 years tons of engines that had been running on non-ethanol blends had tons of trouble with deposits breaking loose and causing issues, especially anything with a carburetor that had built up deposits. The Ethanol broke down the deposits and then they'd get lodged in jets.
Modern engines seem way better at not building up deposits like you said, but things like Direct Injection also didn't become common (at least in the US) till Ethanol was also present.
As a practical matter, it doesn't make a difference because almost everyone occasionally uses detergent gasoline (e.g., when that is the most convenient gas station).
I am definitely not someone with experience in fluid mechanics, but I feel this goes against a layman's understanding of it? Could you provide some quick material on this claim so I could understand it? I already tried searching it but due to my limited knowledge of the subject I couldn't word my search to get the results I wanted.