A few years ago I used to fly a plane that had no mixture lever. It was built in 1948 and had a german engine (Argus, 465 HP).
Design probably from WW2. Mixture and ignition timing fully automatic. No lambda or FADEC stuff and absolutely reliable.
So it would be feasable and reliable also these days even without electronics.
The problem is: most modern (american) piston engines are still 1940's technology with maybe an injection syatem and turbocharger added. And these companies do have some kind of monopoly and absolutely no will to start anything really new.
Cheers, jojodel