Well, the guitar isn't a real instrument, either. You can do so much more on a harp-guitar.

Well, the harp-guitar isn't a real instrument, either, because you can do so much more on a piano.

But the piano isn't a real instrument because you can do so much more on an organ.

Which isn't a real instrument, because you can do so much more on a synthesizer.

But a computer is capable of generating music a synthesizer can't touch.

But a computer isn't a real instrument, either, because the wind puts all other instruments to shame: it makes the most amazing variety of tones, effects and dynamics, using a trillion voices at the same time.

So I guess the only real instrument is the flute.