You make a very valid point. These keyboards have tons of functions, and I bet the majority of owners have only scratched the surface of many of them.
Never mind a Tyros or PSR-S900 -- I've been using a PSR-E433 for eight years, and it has served me well. But for me, I mainly only use styles for the drums, so the various instrumental features for the styles on the advanced models are not important to me. For me, it's the sound and sound creation that are important, and the E433 is quite versatile, even including filter and envelope generator. There are things that I wish it had, such as the ability to set the sustain pedal to sustain both sides of a split keyboard, so I always keep looking out for what the newest models can do, but so far, Yamaha hasn't implemented the main changes I would like to see. Well see what happens with the PSR-E473 when it is finally introduced.