Well, the assertion "people get motivated by money" is questionable. I
remember a psychology paper showing that children draw significantly better
if they are not promised candies to do so (the generalization to any work and
any reward is questionable too).
Anyway, I believe nobody would pretend that the Linux kernel would benefit
from not having any paid developers since, as you wrote, they are the main
contributors today.
But... what is your point? :-) Nobody here pretends that free software
developers should never be paid for their contributions. Stallman does not
say that either. Those contributions are positive whereas proprietary
software developments are negative contributions from rms' freedom point of
view. The former should be encouraged (e.g., with monetary rewards) and the
latter discouraged.
