Are we using the same definition of "expressive"? I play synth, guitar and trumpet, and the trumpet is by far the most expressive of the three, both musically and physically. You have basically all the same options for expression that you do with a human voice (vibrato, dynamics, glissando, etc) plus the expressive techniques offered by the instrument mechanics (for example: half-valving, trills, lip slurs, using a plunger as a LPF).

Sure you need a microphone or contact mic, but again that's just your source of your oscillator. After that, sound design is just sound design. I'm not saying everybody should play electric trumpet, but it's just absurd to make blanket statements like "electric guitar is the most expressive electronic instrument".