if your primary argument is that a keyboard/sampler/turntable does not qualify as an 'instrument' because it is actually a 'machine,' then in order to maintain philosophical consistency, any 'machine' can not be an 'instrument.'

A 'machine' is a too that does some of your work for you. In the sense of machines used to produce music, I would apply this to any instrument where the interaction of the 'musician' and the element of the machine that produces the sound is indirect.

by indirect interaction, I mean that the musician does not physically interact with the mechanism which produces sound.

In an accordion, a human pumps the bellows, but the bellows forces the air through the reeds.

In a piano, a human pushes a button (sound familiar?) and the internal machine raises and lowers a hammer onto a string. Philosophically it's absolutely no different from a keyboard in which a human pushes a button and a sound comes out of the speaker; in both cases, a machine is doing what the human instructed it to do in order to produce a specific sound.

Electric instruments like guitar present sort of a hybrid of direct and indirect; which sound is produced through direct interaction, it is no usually the same sound that comes out the other end.