John McTammany's "Technical history of the player" describes the
long haul to the development of the double-valve pneumatic system,
which took some 20 years after the paper-roll reed organs first
appeared on the market.

Oddly enough, some of the earliest player systems were solenoid-
operated one way or another.  The Schmoele brothers' 1876 patent read
the roll using a brush contact and opened a pneumatic system with a
solenoid (theirs was the very first double-valve pneumatic action).

Another approach was to have a rotating cylinder and use a solenoid
to drop a friction foot onto it to kick the piano action (exactly the
technology of the electric typewriter many decades later); these
instruments were available very early on.  I don't know whether
direct-acting solenoids were tried for note actuation.

All the electrical systems went by the wayside with the advent of
pneumatics.  McTammany credited the development of thin flexible pouch
leather as making the modern valve systems possible, it only coming
onto the market in the late 1880s.  As he said, inventors couldn't make
instruments with non-existent materials!

The switching of a pre-set air supply was the vital thing that made the
player piano possible.  The piano needs a sudden percussive power to
operate it, not a continuous flow, and the force of the power must be
continuously variable to give expressive playing.  The compressible
nature of air is ideal because it allows the force to be built up
before the note operates and be delivered very fast on demand.

The subtle point of the player piano is that the operator controls how
loudly it plays by foot pedals, with the varying suction supply
offering tactile feedback via the pedals so allowing accurate control
of the playing level.  Inside the player's pneumatic reservoir is a
highly non-linear spring so that the harder the operator plays, the
greater the force on the reservoir and the greater the suction.  It's a
clever system, and works beautifully (most player piano owners never
even realise just how clever it is - always a mark of good design!).
For the particular application it's a most appropriate use of
technology and just as effective as modern feedback-style robotic
systems, some of which I believe use pneumatics!

Many, indeed most, "mechanical" music instrument types don't need
interactive tactile feedback but operate to a pre-programmed sequence.
The arguments here are different.

Julian Dyer